Most medications clear within 1 to 5 days, set by the half-life: about 5 half-lives means roughly 97 percent eliminated.
The range is huge: amoxicillin clears in hours, cetirizine about a day, sertraline days, and fluoxetine (Prozac) weeks, which is why it lingers long after the last dose. The stretch factors, per Merck Manual elimination guidance: liver or kidney impairment (those organs do nearly all the clearing), older age, fat-solubility, competing medications, and long-term accumulation. Why the question matters: stopping or switching drugs (the old drug's tail shapes the new one's start), drug testing (detection windows differ from clinical clearance and depend on the test), lingering side effects, and pregnancy safety. The key caution: clearance math never replaces a taper, because withdrawal is about receptor adaptation, not blood levels, and the nervous system can keep adjusting for weeks after the drug itself is gone.
What to do
Common questions
How long does medication stay in your system?
The governing concept is half-life: the time for your body to eliminate half of the drug in your blood. Pharmacology's rule of thumb (per the Merck Manual's drug elimination guidance) is that a drug is effectively cleared after about 5 half-lives, roughly 97 percent gone. Applied to common medications: amoxicillin's half-life is about an hour, so it clears in hours; ibuprofen hours; cetirizine (Zyrtec) about a day; sertraline days; fluoxetine (Prozac) is the famous outlier with a half-life of days to weeks, which is why its effects and interactions linger long after the last pill. What stretches the timeline: liver or kidney impairment (the two organs that do nearly all the eliminating, per Merck), older age (both slow naturally), body composition for fat-soluble drugs, other medications competing for the same liver enzymes, and dose and duration, since long-term use lets some drugs accumulate in tissue. The practical reasons people ask: switching or stopping medications (the old drug's presence shapes the taper and the new drug's start, which is why "when did you last take it" is a real clinical question), drug testing (detection windows differ from clinical clearance and vary by test type: urine, blood, hair), side-effect persistence after stopping, and pregnancy safety. The rule that matters most: never use clearance math to skip a taper, because withdrawal and discontinuation syndromes are about receptor adaptation, not blood levels, and a "clear" drug can still leave your nervous system adjusting for weeks.