How long does a drug stay in your system?
Half-life is the time it takes your body to clear half of a drug from your blood. This calculator uses a single representative elimination half-life for the parent drug in a healthy adult, then applies the standard clinical rule to estimate roughly how long the drug lingers. It gives you a ballpark timeline, not a personal readout.
How it works
A drug falls by half every half-life, so the fraction still in you after n half-lives is (1/2) to the n. That means about 50% is gone after 1 half-life, ~75% after 2, ~94% after 4, ~97% after 5, and ~99% after 6-7. The common clinical rule of thumb is that a drug is essentially cleared after about 4 to 5 half-lives, and on repeat dosing it reaches steady state (levels stop climbing) over that same 4-to-5-half-life window. We multiply the drug's elimination half-life by 5 to get the "mostly out of your system" estimate.
Worked example
Take lorazepam, with an elimination half-life of about 12 hours. After 12 hours roughly half is gone; after 24 hours ~75%; after 48 hours ~94%; and after 5 half-lives (about 60 hours, or 2.5 days) ~97% is cleared. So a single dose is largely out of your system in a little over two days in a typical healthy adult. Fluoxetine is the opposite extreme: the parent has a half-life around 4-6 days, but its active metabolite norfluoxetine runs 4-16 days, so meaningful drug can persist for weeks after the last dose.
Important
Estimate only, for healthy adults — not medical, pharmacist, or poison-control advice, and not valid for children. It multiplies ONE representative elimination half-life by 5. Your real clearance shifts with age, kidney and liver function, dose, how long you have taken the drug, genetics, other medications, and body weight, so your number can be very different. This is NOT a drug-test calculator. What this tool estimates is when the drug leaves your BLOOD. Urine, hair, and saliva tests detect the drug and its metabolites on their own, usually LONGER, timelines that vary by person, dose, duration of use, and lab — a negative estimate here does NOT mean you will test negative. Using it to time or pass a test is unreliable and unsafe. Two traps that make the real time far longer than the number shown: (1) Active metabolites. For some drugs the true "out of your system" time is far longer than 5x the parent half-life — diazepam's metabolite nordiazepam lasts ~100 hours, and fluoxetine's norfluoxetine lasts 4-16 DAYS (why an MAOI needs a 5-week washout after fluoxetine). Sertraline, venlafaxine, and bupropion also have longer-lasting active metabolites, among others — the parent-drug math understates all of them. (2) Organ impairment. Kidney disease dramatically prolongs renally cleared drugs like gabapentin and pregabalin; liver disease prolongs many others. Clearance can take far longer than shown. Never start, stop, or change a medication, and never assume it is safe to take an interacting drug, drive, or breastfeed, based on this estimate. Talk to your prescriber or pharmacist. Sources: StatPearls "Elimination Half-Life of Drugs" (NCBI Bookshelf NBK554498) and "Pharmacokinetics" (NBK557744); FDA Prozac (fluoxetine) prescribing information; DailyMed FDA drug labels. All four verified live and correctly attributed.