Is metabolite a drug?
Active metabolite
An active metabolite results when a drug is metabolized 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 (morphine and "O"-desmethyltramadol respectively) that are stronger than the parent drug 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.
Drug discovery
Many secondary metabolites produced by plants have potential therapeutic medicinal properties. These secondary metabolites contain bind to and modify the function of proteins (receptors, enzymes, etc.). Consequently, plant derived natural products have often been used as the starting point for drug discovery.