Half-life

The time it takes for the amount of a drug in the blood to fall by half.

Half-life is the time required for the concentration of a drug in the bloodstream to drop to half its starting value. It governs how often a medication has to be taken: a short half-life usually means several doses a day, a long one may allow a single daily dose. It also sets how long a drug lingers after the last dose — roughly four to five half-lives are needed before a drug is substantially cleared, which is why stopping a long-half-life medication produces effects that fade slowly rather than abruptly.

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