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L-Tyrosine
L-tyrosine is sold as a daily dopamine, mood, and thyroid booster, and on an ordinary day it does almost nothing: the enzyme that turns it into brain chemicals is already near full, so more raw material changes nothing. The evidence backs one job. On the night you can't reschedule, when an unavoidable stressor drains your brain faster than it rebuilds, two grams beforehand protects the working memory and reaction speed that fail first. A few cents of insurance for the wrong night, not a habit.
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It only works when you're running flat out. Dopamine and noradrenaline are built from tyrosine, and the enzyme that does it sits about three-quarters saturated at rest 1. Add more substrate to a near-full enzyme and nothing speeds up. Cold, sleep loss, and sustained high-stakes attention make those neurons fire fast enough to drain the pool faster than it refills, and only then does extra tyrosine raise output 2. It is a stand-by raw material that matters when the factory is maxed.

The signal is narrow and consistent. Under cold, sleep loss, or heavy multitasking it holds working memory and reaction time that placebo lets slide, for about three hours 3, 4, 5. A military review of the trials graded a weak recommendation for cognitive performance under acute stress 6. The benefit shows up under depletion and fades at rest 7. At rest in healthy young adults it nudges a puzzle or a task-switch and little else 8, 9.

Take it on the wrong night, and empty.

On the nights that carry weight, the split is small but real: by hour four of a stressor your working memory is already eroding, and the trial data show tyrosine preserving a slice of it for about three hours 3. Not transformation. The difference between reading the paragraph twice and catching it the first time, when catching it is the whole job. Most days the stake is zero, and skipping it costs you nothing.

The fine print โ€” when to skip it, and what people get wrong

Bottle claims the trials don't back. Dopamine: only under real stress, never at rest 2. Thyroid: builds T3 and T4, but no trial moves it in healthy people. Depression, ADHD: no antidepressant effect 11; ADHD faded by week six 12.

"It did nothing" usually means a protein meal, no real stressor, or daily use until tolerance built 12. Or you're over 60, where rising doses worsened working memory 10.

Hard stop on MAOI antidepressants and the antibiotic linezolid: a credible hypertensive-crisis risk. Skip it on levodopa, in active hyperthyroidism, in pregnancy, with a melanoma history. Otherwise side effects are mild: nausea, heartburn, headache 13.

References
  1. 1Fernstrom JD, Fernstrom MH (2007). Tyrosine, phenylalanine, and catecholamine synthesis and function in the brain. Journal of Nutrition. link
  2. 2Carlsson A, Lindqvist M (1978). Dependence of 5-HT and catecholamine synthesis on concentrations of precursor amino-acids in rat brain. Naunyn-Schmiedeberg's Archives of Pharmacology. link
  3. 3Mahoney CR, Castellani J, Kramer FM, Young A, Lieberman HR (2007). Tyrosine supplementation mitigates working memory decrements during cold exposure. Physiology & Behavior. link
  4. 4Neri DF, Wiegmann D, Stanny RR, Shappell SA, McCardie A, McKay DL (1995). The effects of tyrosine on cognitive performance during extended wakefulness. Aviation, Space, and Environmental Medicine. link
  5. 5Thomas JR, Lockwood PA, Singh A, Deuster PA (1999). Tyrosine improves working memory in a multitasking environment. Pharmacology Biochemistry and Behavior. link
  6. 6Mahoney CR, Castellani J, Kramer FM, Young AJ, Lieberman HR (2015). Tyrosine for mitigating stress and enhancing performance in healthy adult humans, a rapid evidence assessment of the literature. Military Medicine. link
  7. 7Jongkees BJ, Hommel B, Kuhn S, Colzato LS (2015). Effect of tyrosine supplementation on clinical and healthy populations under stress or cognitive demands โ€” a review. Journal of Psychiatric Research. link
  8. 8Colzato LS, de Haan AM, Hommel B (2014). Food for creativity: tyrosine promotes deep thinking. Psychological Research. link
  9. 9Steenbergen L, Sellaro R, Hommel B, Colzato LS (2015). Tyrosine promotes cognitive flexibility: evidence from proactive vs. reactive control during task switching performance. Neuropsychologia. link
  10. 10van de Rest O, Bloemendaal M, de Heus R, Aarts E (2017). Dose-dependent effects of oral tyrosine administration on plasma tyrosine levels and cognition in aging. Nutrients. link
  11. 11Gelenberg AJ, Wojcik JD, Falk WE, Baldessarini RJ, Zeisel SH, Schoenfeld D, Mok GS (1990). Tyrosine for depression: a double-blind trial. Journal of Affective Disorders. link
  12. 12Reimherr FW, Wender PH, Wood DR, Ward M (1987). An open trial of L-tyrosine in the treatment of attention deficit disorder, residual type. American Journal of Psychiatry. link
  13. 13Morgan CA, Rasmusson A, Pietrzak RH, Coric V, Southwick SM (2009). Relationships among plasma dehydroepiandrosterone and dehydroepiandrosterone sulfate, cortisol, symptoms of dissociation, and objective performance in humans exposed to underwater navigation stress. Biological Psychiatry. link
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