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Half-life is the time required for the drug’s concentration in the body to fall by half. This measure shows how long the drug remains in the body and can keep producing effects as it’s gradually eliminated. Knowing the half-life helps explain dosing schedules: drugs with short half-lives need more frequent dosing to maintain therapeutic levels, while those with long half-lives can be dosed less often. This concept is different from potency, which is about how much drug is needed to achieve an effect; and from duration, which describes how long the overall effect lasts and can be influenced by factors beyond just elimination. Therapeutic index refers to the safety margin between effective and toxic doses. The half-life specifically reflects pharmacokinetics—the rate the body eliminates the drug—informing how long it stays active and when levels drop enough to reduce effect.