Four ways a supplement moves a drug. It can speed clearance: St John's wort trains the liver to burn through drugs faster, so within two weeks cyclosporine, HIV drugs and the contraceptive pill drop below their working level ¹. It can block clearance: one glass of grapefruit juice disables a gut enzyme for a day, so a normal simvastatin dose hits ten times as hard ². It can bind the drug in your stomach: calcium and iron clamp thyroid tablets and fluoroquinolone antibiotics into a clump that never absorbs ³. Or it can pile onto the same chemistry: 5-HTP and SAMe add to what an antidepressant does to serotonin ⁴.

These aren't case-report guesses. Volunteers on the HIV drug indinavir added St John's wort; levels fell 57%, and the trough that keeps the virus down fell 81% ⁵. Two heart-transplant patients started it for low mood and rejected their grafts within weeks, both recovering once they stopped ⁶. Gram-dose niacin plus a statin, tested in 25,673 patients: no heart benefit, four times the muscle damage ⁷. Calcium with levothyroxine cuts absorption twenty to fifty percent; a four-hour gap erases it ³.

This is a screening habit, not a daily routine. Four moves cover most of it.

Who this is really for. On no prescriptions, it's background reading. The stakes land on four groups: anyone on a narrow-margin drug, on levothyroxine, on an antidepressant eyeing a mood supplement, or on five-plus prescriptions, where roughly one older adult in seven risks a major interaction each year ⁸.

Done wrong, the failure is quiet. A transplant patient feels nothing change for three weeks; the rejection paperwork arrives at the fourth clinic visit. A thyroid patient feels foggy for months because the morning calcium and the pill shared a stomach. Prevention costs one list and one sentence.