active metabolite

We often picture metabolism as the body 'breaking down' a drug into harmless debris. An active metabolite breaks that picture: it is a product of metabolism that still works as a drug, sometimes just as well as — or even better than — the original molecule.

Active metabolites form when biotransformation produces a compound that retains affinity and efficacy at the drug target. They can prolong or intensify a drug's effect, and they may have their own half-life and side-effect profile. Diazepam yields the long-acting active metabolite nordiazepam, and fluoxetine forms norfluoxetine, which contributes to its lengthy duration of action.

Active metabolites matter clinically because they shape how long and how strongly a drug acts, complicate dosing, and can accumulate when elimination is impaired — for instance, morphine's active metabolite morphine-6-glucuronide can build up in kidney failure and cause excessive sedation.