How long does medication stay in your system?

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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.

How long does medication stay in your system?

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