Medicines vary in their half-lives -- that is, how long it takes for half the medicine to become undetectable in the bloodstream. It takes 5 half-lives for a medicine to become undetectable in blood. Medicines also vary in what other tissues besides the blood they penetrate. Lithium, for example, has a half-life of about a day, so, 5 half-lives means 5 days until a lithium level becomes zero. But, as a salt, lithium can become highly concentrated in brain (fat) and bone and can stay there for weeks and weeks...yet blood levels may be undetectable. Drugs also can metabolites -- meaning, breakdown compounds that linger in the body even when the actual drug (parent compound) is gone. A good example is Prozac, whose half-life is only 3-5 days but its metabolite, norfluoxetine, is 5-7 days (so, 7 days x 5 half lives = 35 days...which is why there is no need to taper off Prozac, it will auto-taper). Abilify has a half-life of about 75 hours, so it will last about 2 weeks once someone stops taking it (again, meaning there isn't much need to taper down the dose when stopping...it will auto-taper).
Pristiq and Lamictal both have half-lives of about a day, so 5 half-lives = 5 days. Saphris has a half-life of 12 hours, so 5 half lives is about 3 days. Klonopin's half-life is about 30-40 hours, so, times 5 means 150-200 hours.
You are encouraged to report negative side effects of prescription drugs to the FDA. Visit the FDA MedWatch website or call 1-800-FDA-1088.
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