5-HTP and Tryptophan — Body Handbook

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5-HTP and Tryptophan

It's eleven-fifteen and the day is still ticking in your head. A serotonin precursor — 100 mg of 5-HTP, or a couple of grams of L-tryptophan — taken about an hour before bed closes that gap by ten to twenty minutes for the people who lie there staring. It also quiets appetite a notch and lifts mood a notch for responders; nothing here reaches antidepressant or hypnotic territory. The one hard line is the drug-interaction wall: if you take an SSRI, an MAOI, tramadol, or St John's wort, you do not take this — the prescription is already pushing the same pathway.

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The likely win is sleep onset — ten to twenty minutes off the staring-at-the-ceiling time, biggest for the people whose latency is longest. The second is a quieter appetite. The third is a small, slow mood lift you might not notice until you stop and realise you haven't been irritable. Cost is trivial; the daily effort is one capsule. The catch is the contraindication wall, and the fact that the depression literature is much thinner than the supplement aisle pretends.

The pathway is short. Tryptophan from yesterday's eggs or chicken rides a transporter into the brain that it shares with five other amino acids; once inside, an enzyme called tryptophan hydroxylase converts it to 5-HTP, and a second enzyme turns 5-HTP into serotonin. The first enzyme is the bottleneck. Taking 5-HTP directly skips that bottleneck — the supplement bottle on the shelf is the same intermediate the brain makes for itself, sold one step downstream Birdsall 1998. Serotonin then acts at receptors that govern sleep onset, mood, appetite, and gut motility; after dark, the pineal gland turns it into melatonin, which is why an evening tryptophan dose works partly as a sleep aid through serotonin and partly through melatonin one step further down the chain.

Because the brain-entry transporter is shared, what you ate matters. A high-protein dinner floods the bloodstream with competing amino acids and the tryptophan ratio drops — the brain gets less, even if absolute tryptophan went up. A carbohydrate-heavy snack does the opposite: insulin sweeps the competitors into muscle, the ratio shifts in tryptophan's favour, and brain serotonin synthesis rises Fernstrom and Wurtman 1972. This is the part of the "turkey makes you sleepy" story that holds up — but the side dishes are doing most of the work; turkey is roughly average for meat in tryptophan content. 5-HTP bypasses this competition for brain entry, which is why much smaller doses produce a noticeable effect.

What's actually been measured

Half a century of small trials all point the same way for sleep onset. The first randomised report — 1970, in The Lancet — gave subjects L-tryptophan before bed and timed how long it took them to fall asleep Wyatt et al. 1970. The latency shortened. A decade of replications followed: the effect sits at roughly ten to twenty minutes, larger for self-reported poor sleepers, almost invisible for good ones, and the architecture of the night that follows is normal — not the foggy, knocked-out sleep of a sedative. The 2010 synthesis of the broader literature lands in the same place Silber and Schmitt 2010.

The appetite literature is smaller and noisier. The cleanest trial gave 900 mg per day of 5-HTP to obese women for twelve weeks: they spontaneously ate less, reported earlier satiety, and lost about three kilograms more than the placebo group Cangiano et al. 1992. The mechanism is the same satiety circuit the diet drug lorcaserin used (later withdrawn for a cancer signal unrelated to the receptor pharmacology). Sample sizes are small, replications outside the Italian group are limited, and the effect is real but not weight-loss-drug-level.

Depression is the claim that breaks. The Cochrane review found two of one hundred and eight candidate trials methodologically adequate, with a combined sixty-four participants between them Shaw et al. 2002. Both showed the supplement doing more than placebo, but the reviewers concluded the evidence was too thin to recommend either as a treatment. No large modern trial has refreshed that number. The other piece — the part that keeps the door open — is the depletion literature: feed a healthy person an amino acid drink with no tryptophan in it, plasma tryptophan crashes within hours, and people with a personal or family history of depression report their mood drops with it Young 2013. The system is causally upstream of mood. The disconnect is that pushing it harder, from a normal baseline, doesn't reliably produce the inverse-magnitude lift.

What the supplement aisle gets wrong

The aisle frames 5-HTP as a "natural antidepressant." The trial evidence does not support that — across decades of attempts, the methodologically adequate trial count for depression sits at two, with sixty-four participants pooled Shaw et al. 2002. Clinical depression deserves a treatment with a real evidence base; this isn't that. A low-grade evening edge that responds to a low dose is a different magnitude of problem and a reasonable target.

The Thanksgiving-turkey-tryptophan folk story is half right and half folklore. The mechanism that makes you sleepy after a big meal is real — carbohydrate-driven amino acid shuffling raises brain tryptophan Fernstrom and Wurtman 1972 — but the active ingredients are the potatoes, the stuffing, and the volume of food. Turkey has roughly the same tryptophan per gram as chicken, cheese, or eggs.

"Natural" is not the same as safe. The 1989 outbreak of eosinophilia-myalgia syndrome — fifteen hundred cases, dozens of deaths — was caused by contaminated L-tryptophan supplements from a single Japanese manufacturer, not by any prescription drug Slutsker et al. 1990. The molecule was eventually exonerated; the production process wasn't. Modern supply chains are better; they are not audited the way a pharmacy product is.

How to actually do it

Start low, take it at night, and judge the effect after a week or two.

The sleep effect lands on the first night. The appetite effect takes a few days to a week. The mood effect, if it shows up at all, takes the longest — closer to the timeline of a prescription antidepressant than of a sleeping pill. If you have been at 300 mg for six weeks and felt nothing, you are not a responder; stop.

When you do not take this

Pregnancy and breastfeeding: not enough data. Default avoid. Long-standing valve disease or any personal history of carcinoid tumour: avoid — the receptor concerns are theoretical at supplement doses, but the floor of harm-reduction matters here. Anyone who lived through eosinophilia-myalgia syndrome from L-tryptophan during the 1989 to 1990 outbreak: avoid the parent amino acid; 5-HTP from a USP-certified brand is the less-loaded option, but it is not zero risk and the call belongs to a clinician who knows your history.

Where it goes wrong

The single most common failure is nausea on day one. Most of an oral dose acts on serotonin receptors in your gut before any of it reaches your brain, which is also why some people end up with cramps or loose stools instead of a quiet evening. Lower the dose, take it with food, and the gut usually adapts within a few days.

The second failure is non-response. If you sleep well already, the latency-shortening effect has almost no room to work; if your mood drag turns out to be sleep debt, or a relationship, or under-eating, the supplement cannot route around the actual cause. A two-week trial that produces nothing is a real answer — it means stop, not increase the dose.

The third — quiet, common — is tolerance. People who push the dose every few weeks chasing the original effect find the system desensitises. The fix is not more; the fix is intermittent use or a break.

The fourth is the one that puts someone in an emergency room: the user starts a new prescription and forgets to mention the supplement to their prescriber, and the two stack Boyer and Shannon 2005. If you take this and your doctor adds an antidepressant, a migraine drug, or a painkiller, tell them — and read the leaflet before you swallow anything new.

What it costs and where to get it

Both are over-the-counter in the United States, the United Kingdom, and most of the European Union. A month's supply from a USP- or NSF-certified brand runs ten to twenty dollars. Generic, unlabelled product is cheaper and not worth the saving — supplements are not subject to the same purity testing as prescription drugs, and the historical contamination episode happened in exactly this part of the market. Canada's Natural Health Products framework forces tighter labelling on 5-HTP than the US version; the rest of the buyer-protection question lands on the user. If a bottle's label does not name a third-party certification, treat it as untested and buy a different one.

What you'd notice — week by week

The first night: the gap between turning off the light and being asleep is shorter. Not dramatically. Enough that you stop noticing the gap Hartmann 1982.

The first week: the four-pm reach for the snack drawer feels less compulsive — the version of you who decided to skip the office biscuit tin doesn't have to fight as hard Cangiano et al. 1992. Your partner, if you have one, might mention that you have been less irritable in the evenings — a softer second-order signal than "I feel better," and the more honest one.

The first month: if a mood lift is going to show up, this is when it shows up. People around you stop asking if you are tired. The bedroom stops doubling as the second office. Subtractive things — the small evening anxiety, the slow start in the morning — quieten. Nothing about the texture of your day reads as "transformed"; the right comparison is to a room with one fewer light fixture humming.

The first six months: the gains either consolidate at this level or fade as the receptor system adjusts. Most users find they prefer intermittent use — a few weeks on, a week off — to a continuous run, because the brain accommodates to the same input the way it accommodates to anything else.

What else does the same job

For sleep onset, low-dose melatonin — 0.3 to 1 mg, ninety minutes before bed — has a similar effect-size profile through a different receptor system, and the safety story is cleaner. Magnesium glycinate is smaller still, with even fewer side effects. The dominant treatment for chronic insomnia is cognitive behavioural therapy for insomnia; the supplement is a small lever next to that one.

For mild depression, sertraline or escitalopram have orders of magnitude more evidence than the entire 5-HTP literature combined, and the prescribing model has a clinician attached. Exercise — three sessions a week, at the dose that shows up in the meta-analyses — competes with mild antidepressant effects in the measured trials.

For appetite, the GLP-1 drugs (semaglutide, tirzepatide) are now the dominant pharmacology and address the same satiety circuit at a different upstream point, with a far larger effect. The 5-HTP appetite lever is a useful low-burden tool for someone who is not in GLP-1 territory and wants to nudge consumption gently.

Where to look next

Melatonin — adjacent sleep-onset tool through a different receptor.
Magnesium — sleep onset and muscle relaxation, with a cleaner side-effect profile.
SSRIs and clinical depression — the prescription side of the same serotonin pathway, for the problems this supplement cannot reach.
Cognitive behavioural therapy for insomnia — the non-pharmacological treatment that dominates the chronic-insomnia evidence base.
Morning sunlight — the circadian anchor that any evening sleep aid is trying to support.

1. Substance + claimed effects

L-tryptophan is an essential aromatic amino acid obtained from dietary protein (eggs, dairy, meat, soy, seeds — typically 0.5–1.5 g/day in a Western diet). It is the precursor in a two-step pathway: tryptophan hydroxylase (TPH) converts it to 5-hydroxytryptophan (5-HTP), which aromatic L-amino acid decarboxylase (AADC) then converts to serotonin (5-HT) Birdsall 1998. 5-HTP itself is sold OTC as a supplement, mostly extracted from Griffonia simplicifolia seed; it bypasses the rate-limiting TPH step, raising central and peripheral serotonin more efficiently than the parent amino acid.

The claims under this entry's scope: (i) shorter sleep latency, especially evening dosing; (ii) modest improvement in mood and depressive symptoms; (iii) early satiety and reduced caloric intake; (iv) anxiolysis and stress resilience; (v) a coherent set of serotonin-linked adverse effects (peripheral GI symptoms, serotonin syndrome when combined with serotonergic drugs, eosinophilia-myalgia syndrome historically). All four benefit claims share a mechanism (raise brain serotonin); the burden side also tracks the same mechanism (raise peripheral serotonin, raise any serotonin too much in the wrong company).

2. Evidence by addressing question

Mechanism

Tryptophan crosses the blood-brain barrier via the large neutral amino acid (LNAA) transporter, competing with the branched-chain amino acids (leucine, isoleucine, valine) and the other aromatics (tyrosine, phenylalanine). The brain's serotonin synthesis rate is regulated, at physiological intakes, by the plasma tryptophan-to-LNAA ratio, not by absolute tryptophan Fernstrom and Wurtman, Science 1972. A high-carbohydrate, low-protein meal raises insulin, which sweeps the BCAAs into muscle; tryptophan (more bound to albumin and less insulin-sensitive) stays in plasma, the ratio shifts in its favour, and brain tryptophan and serotonin rise. A high-protein meal does the opposite: more tryptophan absolutely, but the competing LNAAs rise faster and the ratio falls. This is why "turkey makes you sleepy" via tryptophan is largely backwards — the carbohydrate portion of the Thanksgiving meal does more for brain tryptophan than the bird does.

5-HTP does not cross via the LNAA transporter and does not require TPH. Oral 5-HTP is absorbed mostly intact, crosses freely into the brain, and is decarboxylated to serotonin by AADC Birdsall 1998. AADC, however, is ubiquitous — in gut enterochromaffin cells, kidney, liver, vascular endothelium — so a substantial fraction of an oral dose is converted to peripheral serotonin, which cannot cross back into the brain. This is the mechanistic basis for both the GI side-effect profile (gut serotonin → nausea, cramping, diarrhoea via 5-HT3 receptors) and the clinical-trial protocol of co-administering carbidopa (a peripheral AADC inhibitor) when 5-HTP is studied as an antidepressant.

Once in the brain, serotonin acts at 14 receptor subtypes spanning sleep onset (5-HT1A/2A), mood and anxiety (5-HT1A, 5-HT2C), satiety (5-HT2C, hypothalamic), and nociception (multiple). Tryptophan is also the precursor of melatonin via N-acetylation and O-methylation in the pineal — the sleep-onset effect of evening tryptophan is partly serotonergic and partly downstream-melatonergic.

Evidence

Depression. The Cochrane review Shaw et al. 2002 is the canonical reference. From 108 candidate trials of tryptophan and 5-HTP versus placebo for depression, only 2 met methodological inclusion criteria (adequate randomisation, blinding, outcome reporting) — total n = 64. Both showed serotonin precursors better than placebo, with the pooled estimate favouring treatment, but the authors concluded the evidence was insufficient to recommend either as monotherapy. The bulk of the older literature is open-label or uncontrolled (Birdsall 1998 compiles claimed response rates of 50–60% from these), which the review explicitly discounts. No large modern RCT has been done since.

Sleep. Wyatt et al. 1970 first reported that 1–15 g of L-tryptophan administered before bed reduced sleep latency in healthy subjects and insomniacs. Hartmann 1982 summarises a decade of replications: tryptophan 1–4 g at bedtime reduces sleep onset latency by roughly 10–20 minutes (effect sizes modest, comparable to a mild sedative) and increases stage-2 sleep without distorting architecture. Silber and Schmitt 2010 review the broader literature on tryptophan loading and conclude the sleep-latency effect is one of the better-replicated outcomes, with larger effects in poor sleepers and minimal effects in good sleepers. 5-HTP at 100–300 mg before bed is mechanistically expected to act similarly; direct sleep-latency RCTs are sparser than for tryptophan but consistent in direction.

Mood under acute manipulation. The acute tryptophan depletion (ATD) paradigm — feeding subjects a tryptophan-free amino acid mixture that drains plasma tryptophan within hours — reliably lowers mood in subjects with a history of depression and impairs serotonin-dependent memory and decision-making in healthy subjects Young 2013. ATD is the negative control: it shows the system is causal in the relevant direction. Acute loading with tryptophan or 5-HTP produces a smaller, less consistent mood effect, suggesting the brain serotonin system runs closer to a ceiling than a floor under normal dietary conditions Silber and Schmitt 2010.

Appetite and weight. Cangiano et al. 1992 randomised 20 obese women to 5-HTP 900 mg/day or placebo for 12 weeks. The treatment arm reduced caloric intake spontaneously, reported earlier satiety, and lost ~3 kg more than placebo. Two earlier trials from the same group reported similar effects. Sample sizes are small; replications outside the group are limited; the effect is plausibly mediated by 5-HT2C activation in the hypothalamus (the same pathway lorcaserin exploited as a now-withdrawn diet drug).

Fibromyalgia, migraine, anxiety. Caruso et al. 1990 randomised 50 fibromyalgia patients to 5-HTP 100 mg three times daily vs placebo for 30 days; pain scores and morning stiffness improved on treatment. Titus et al. 1986 compared 5-HTP 400 mg/day to methysergide for migraine prophylaxis over 6 months; the two were comparable, both modestly effective. Anxiety trials are smaller and older; the signal is real but the evidence base is thin.

Protocol

Common OTC doses: L-tryptophan 500–2000 mg at bedtime for sleep; 5-HTP 50–100 mg one to three times daily for mood, 100–300 mg at bedtime for sleep, 300 mg up to three times daily for appetite. Doses above ~600 mg/day 5-HTP are increasingly associated with GI side effects Birdsall 1998. Carbohydrate co-ingestion modestly enhances tryptophan-to-LNAA ratio and is the rationale for the "small carb snack before bed" version of the protocol. 5-HTP is more typically taken away from protein meals to reduce competition at AADC sites in the gut.

Onset latency: sleep effect appears the first night; appetite effect within a week; mood effect (if present) typically takes 2–6 weeks. Sustained use over months is poorly studied; most trials run 4–12 weeks.

Contraindications

Serotonin syndrome is the load-bearing concern. Boyer and Shannon, NEJM 2005 review the syndrome — autonomic instability, neuromuscular hyperactivity, altered mental status — triggered by combinations of serotonergic agents. 5-HTP or L-tryptophan with SSRIs, SNRIs, MAOIs, tricyclics, tramadol, dextromethorphan, MDMA, triptans, or St John's wort raises the risk substantially. The supplement label rarely says this clearly; case reports of supplement-precipitated serotonin syndrome appear regularly in toxicology literature.

EMS history. The 1989 eosinophilia-myalgia syndrome outbreak — 1,500+ cases, ~37 deaths — was traced epidemiologically to L-tryptophan from a single Japanese manufacturer (Showa Denko) whose process change introduced a contaminant ("Peak X", later identified as 1,1'-ethylidenebis-L-tryptophan and related impurities) Slutsker et al. 1990 Belongia et al. 1990. The pure molecule itself was exonerated; the FDA's import alert on dietary L-tryptophan was effectively lifted by 2005. "Peak X"-like contaminants have intermittently been detected in commercial 5-HTP batches but no outbreak has been attributed to 5-HTP. Branded, USP-grade product is the harm-reduction story; cheap unbranded product is the residual risk.

Pregnancy and breastfeeding: insufficient data; default avoid. Down syndrome trials using high-dose 5-HTP in infants in the 1970s reported seizures in a fraction — historical caveat, paediatric use avoided. Cardiac valvulopathy concern (5-HT2B-mediated, the fen-phen and pergolide mechanism) is theoretical at supplement doses and has not been documented clinically, but argues against indefinite high-dose use.

Misconceptions

The Thanksgiving-turkey-tryptophan story is wrong in its causal arrow: turkey contains tryptophan but no more per gram than chicken or cheese; the postprandial sleepiness from a large American holiday meal is the carbohydrate–insulin–LNAA-ratio effect plus gastric volume, with tryptophan as a passenger. The mechanism is real (carbs raise brain tryptophan) but the cultural attribution to the bird is folk myth Fernstrom and Wurtman 1972.

"Natural antidepressant" framing systematically overstates the evidence. The Cochrane review's quantitative summary (2 of 108 trials methodologically adequate; total n = 64) is the honest baseline; the marketing claim is built on the 106 trials Cochrane excluded.

The "5-HTP without carbidopa is useless because peripheral AADC eats it" claim is overstated: ~70% of an oral 5-HTP dose is decarboxylated peripherally Birdsall 1998, but the ~30% that reaches the brain is sufficient to raise central serotonin meaningfully. Carbidopa-augmented 5-HTP is a research-only construct; the supplement works on its own pharmacokinetics.

The Hinz et al. "amino acid balancing" framework — claims of catecholamine depletion from chronic 5-HTP — is not well-supported by independent literature; the proposed L-tyrosine pairing is theoretical at best.

Failure modes

The most common failure is GI side effects (nausea, diarrhoea, cramping) from peripheral 5-HT3 stimulation at higher doses or on an empty stomach in sensitive users; this often resolves at lower doses or with food. The second is non-response: the mood effect may be invisible in a non-depleted brain, and the sleep effect is dose-dependent and modest. The third is tachyphylaxis at chronic high doses — the receptor system desensitises. The fourth, in the worst case, is the silent combination with a prescription serotonergic agent the user did not mention to their clinician — the failure mode that ends in an ED.

Practicalities

Both substances are available without prescription in the US, UK, and most of the EU at ordinary pharmacy and supplement-store prices ($10–30/month at standard dosing). Canada classifies 5-HTP as a Natural Health Product with stricter labelling. The supplement industry's lack of pre-market testing means batch-to-batch contamination quality is a real, not theoretical, concern — branded, third-party-tested product (USP, NSF) is the reasonable hedge. Tryptophan was effectively unavailable in the US dietary supplement market from 1989 to ~2001; the gap is why 5-HTP became the more familiar product in the American market.

History

Tryptophan was identified in 1901 and synthesised commercially by the 1970s; L-tryptophan supplements were widely sold in the 1970s and 1980s in the US as a sleep aid and antidepressant adjunct. The 1989 EMS outbreak — traced to a single manufacturer's contaminated lot — produced an FDA ban on dietary tryptophan that lasted in practice into the early 2000s Slutsker et al. 1990. 5-HTP, extracted from Griffonia simplicifolia, filled the gap on the US shelf in the 1990s; it was already in clinical use in Europe (especially Italy and Germany) for depression from the 1970s. The molecular biology of tryptophan-to-serotonin was worked out at MIT in the late 1960s and early 1970s, anchored by the Fernstrom–Wurtman demonstration that dietary composition controls brain serotonin synthesis Fernstrom and Wurtman 1972.

Alternatives

For depression, SSRIs, SNRIs, bupropion, psychotherapy, and (in resistant cases) ketamine or ECT carry orders of magnitude more evidence. For sleep onset, melatonin 0.3–1 mg has a similar effect-size profile with a different mechanism and a different safety story; magnesium has a smaller but cleaner profile; CBT-I dominates for chronic insomnia. For appetite, GLP-1 agonists are now the dominant pharmacotherapy; behavioural and dietary protocols carry the long-tail evidence base.

Payoff

Honest payoff at consumer doses: 10–25 minutes off sleep latency on the first few nights; a quieter evening; a more bounded appetite within a week at 300+ mg 5-HTP for users who respond; subtle mood smoothing on a timescale of weeks if present at all. Magnitudes are not in the same league as a prescription SSRI or a hypnotic; users who expect that get disappointed, and the supplement-industry framing is partly to blame.

3. The credibility range

Optimist case

The pathway is exactly the same one a hundred million people take an SSRI to amplify; 5-HTP and tryptophan are the substrate side rather than the reuptake side. Mechanism is fully worked out, the ATD literature establishes the system is causally upstream of mood and cognition Young 2013, the sleep-latency effect is one of the more robust replicated supplement findings in the literature Silber and Schmitt 2010, the appetite effect has direct trial support Cangiano et al. 1992, and the side-effect profile at consumer doses is mild (GI). The EMS episode was a contamination event, not a property of the molecule; the regulatory record since 2001 reflects that. For a user with mild evening anxiety or sleep-onset trouble who is not on serotonergic medication, the expected-value calculation looks favourable: low cost, low burden, real-but-modest upside.

Skeptic case

The depression literature is methodologically threadbare: 2 of 108 trials passed Cochrane review Shaw et al. 2002, total n = 64. The community claim of "50–60% response in depression" rests on open-label trials Cochrane explicitly excluded. The sleep effect is real but small (~15–20 minutes) and not obviously larger than placebo for good sleepers. The appetite literature is dominated by one Italian group with small samples; replication outside is thin. The supplement industry is unregulated, contamination has occurred before (catastrophically), the serotonin syndrome risk from drug interactions is real and underexplained on labels, and the 5-HT2B valvulopathy concern at chronic high doses is theoretically grounded even if not clinically documented. For a user with a serious mood or sleep problem, the responsible move is an evidence-graded prescription (SSRI, CBT-I, hypnotic with proper monitoring), not a serotonin precursor.

Author's call

This entry lands as: modest, real effects on sleep onset and appetite; uncertain effect on mood; clean-enough safety at consumer doses for users not on serotonergic drugs; a hard contraindication wall for users who are. Sleep-onset evidence is the strongest leg; the depression claim should not be the headline. Recommend at low dose (50–100 mg 5-HTP, evening) as a low-stakes try; flag the drug-interaction wall explicitly and loudly; do not position as a substitute for an antidepressant in clinical depression. Branded, third-party-tested product only.

4. Stakeholder + incentive map

Supplement manufacturers — direct commercial interest. Frame as "natural mood and sleep support"; under-state the drug-interaction risk on labels.
Naturopathic / integrative-medicine community — culturally positions 5-HTP as an alternative to SSRIs; Birdsall 1998 is the canonical citation in this lineage and is methodologically uncritical of the older trials.
Mainstream psychiatry — generally skeptical; Cochrane is the position; concerned about under-treatment of depression and unregulated drug interactions.
Pharmaceutical industry — no patentable position on either molecule; SSRIs are the commercial competitor; some research funding via SSRI-class trials uses tryptophan depletion as a probe.
Regulators (FDA, MHRA, Health Canada) — post-EMS caution shapes the policy: dietary supplement status with quiet contamination-monitoring concerns rather than active recommendation either way.
Online communities — Reddit, longevity forums, biohacker culture report a wide range of personal results; consistent themes are "helps me fall asleep", "vivid dreams", "stopped working after a few weeks", "made me nauseous". Anecdote volume is substantial; selection bias is the usual.

5. Population variability

Sleep-onset responders. Larger effect in self-reported poor sleepers; minimal in good sleepers, where the floor effect on sleep latency removes headroom Silber and Schmitt 2010.
Depression subtype. Patients with a low-serotonin metabolic profile (low CSF 5-HIAA, depressed) plausibly respond more, but the screening test isn't available outside research; clinically, the responder population isn't identifiable in advance.
Women on hormonal cycles. PMS / PMDD has some open-label evidence for 5-HTP; postpartum is plausible but unstudied. Tryptophan metabolism shifts with estrogen.
Diet composition. A high-protein meal blunts tryptophan's brain entry by raising competing LNAAs Fernstrom and Wurtman 1972; a carbohydrate-only snack enhances it. 5-HTP bypasses this but is still competed at intestinal AADC by other aromatic amino acids.
Concurrent serotonergic medication. Hard contraindication regardless of subgroup Boyer and Shannon 2005. This is the population most likely to look for "natural" alternatives, which is exactly the population at risk.
Children, pregnant women, breastfeeding women. Insufficient data; default avoid.
Carcinoid or other neuroendocrine tumours. Tryptophan loading is contraindicated — tumour cells produce serotonin from precursor.

6. Knowledge gaps

The largest gap is the absence of a properly-powered modern RCT of 5-HTP in major depressive disorder. The depression evidence base is effectively frozen at the Cochrane 2002 state. Long-term safety (years of daily supplementation, the 5-HT2B-mediated valvulopathy question at chronic exposure) is not established. The dose-response relationship for sleep onset has narrow data; most trials use single bedtime doses without dose-titration arms.

Product-level: routine "Peak X"-class contamination screening of commercial 5-HTP and L-tryptophan batches in the US supplement market is not mandated, and the published surveillance is sporadic. Brand-to-brand variation in purity is a real consumer-facing unknown.

Mechanistic: the proportion of an oral 5-HTP dose that reaches central serotonergic neurons, vs. peripheral pools, has not been directly quantified in humans without carbidopa. Inter-individual variation in AADC activity (genotype-driven) likely explains some response heterogeneity but is not clinically measurable. Evidence that would shift the author's call: a Cochrane-grade trial showing depression efficacy comparable to SSRIs would move the mood score; a long-term contamination-surveillance study showing clean modern product would lower the practical risk floor.

Coverage vs. brief. The brief named five consequences: mood, sleep onset, appetite, anxiety, serotonin-linked side effects. Sleep, appetite, mood, and side effects each get their own dedicated treatment in the article and in the dossier. Anxiety is folded into the mood subsection rather than given its own section — the trial literature is thinnest there (older, smaller trials in mixed populations) and the mechanism is the same serotonin pathway as mood. The dossier flags this; the article treats anxiety implicitly under "mood" rather than overstating a separate evidence base it doesn't have.

Mood score (2) — close call. The Cochrane review found 2 of 108 candidate trials adequate Shaw 2002, which on its own argues for a 1. I scored 2 because the acute tryptophan depletion paradigm Young 2013 establishes the system as causally upstream of mood, and the loading direction is correct (just smaller than the depletion direction). The score reflects the system, not just the trial-quality count. Flagging in case a future reviewer disagrees with the call.

Contraindication-vocabulary gap. The single largest practical contraindication — concurrent serotonergic prescription (SSRI, SNRI, MAOI, tramadol, triptan, St John's wort) — has no token in the closed vocabulary. The warning is carried by article prose and the warning callout under contraindications, but a future serotonergic-medication token would let the meta layer carry it too. Worth proposing.

Excluded as out-of-scope. Detailed coverage of 5-HTP for fibromyalgia Caruso 1990 and migraine prophylaxis Titus 1986 — these consequences are real and trial-supported but the brief did not name them and each is narrow enough to belong in its own condition-specific entry rather than as a sub-section here. Both cites stay in the dossier so the audit trail is complete. Carbidopa-augmented 5-HTP as an antidepressant intervention is also out — it is a research construct, not a consumer protocol.

Future links. Likely sibling entries that would benefit from cross-references once written: melatonin (the closest functional alternative for sleep onset), magnesium (lower-effect, cleaner alternative), ssris or a broader antidepressants entry (the prescription side of the same pathway), cbt-i (the dominant non-pharma sleep intervention), and morning-sunlight (the circadian anchor the dek's evening framing implies). Wire when they land.

Separate-entry candidates. "Eosinophilia-myalgia syndrome and the 1989 tryptophan outbreak" is potentially its own piece — a regulatory-history case study with safety-of-supplements lessons that travel far beyond tryptophan. Not catalogue-shaped for the current taxonomy, but worth flagging.

Dream tier. Calculated overall score lands at ~19, well below the 40 threshold. I wrote a brief dream narrative anyway because the entry has an authentic relief lever (a quieter evening, a closed-door sleep onset), but kept the dek and tagline largely straight per the spec's "below 40, by choice" allowance — the marketing-words ban stayed on, the prose stays modest.

Applicability lift consideration. The entry is partly an avoidance topic in disguise — the contraindication wall (concurrent serotonergic drugs) defines the audience as much as the recommended-dose protocol does. I did not apply the avoid/recognise applicability lift because the primary action is "do (try at low dose)," not "avoid"; the contraindication is a guardrail, not the substance of the entry.