Active metabolite

An active metabolite is an active form of a drug after it has been processed by the body.

Prodrugs
Sometimes drugs are formulated deliberately so they will break down inside the body to form the active drug, these are called prodrugs. The reason for this may be because the drug is more stable during manufacture and storage as the prodrug form, or because the prodrug is better absorbed by the body or has superior pharmacokinetics (e.g., lisdexamphetamine).

Metabolites of drugs
Another kind of active metabolite is when a drug is broken down by the body into a modified form which continues to produce effects in the body. Usually these effects are similar to those of the parent drug but weaker, although they can still be significant (see e.g. 11-hydroxy-THC, morphine-6-glucuronide). Certain drugs, such as codeine and tramadol have metabolites that are stronger than the parent drug (morphine and O-desmethyltramadol respectively)  and in these cases the metabolite may be responsible for much of the therapeutic action of the parent drug. Sometimes however metabolites may produce toxic effects and patients must be monitored carefully to ensure they do not build up in the body. This is an issue with some well known drugs such as pethidine (meperidine) and dextropropoxyphene.