Biotransformation is the process by which a substance changes from one chemical to another (transformed) by a chemical reaction within the body. Metabolism or metabolic transformations are terms frequently used for the biotransformation process. However, metabolism is sometimes not specific for the transformation process but may include other phases of toxicokinetics.
Biotransformation is vital to survival because it transforms absorbed nutrients (food, oxygen, etc.) into substances required for normal body functions. For some pharmaceuticals, it is a metabolite that is therapeutic and not the absorbed drug.
For example, phenoxybenzamine, a drug given to relieve hypertension caused by pheochromocytoma, a kind of tumor, is biotransformed into a metabolite, which is the active agent.
Biotransformation also serves as an important defense mechanism since toxic xenobiotics and body wastes are converted into less harmful substances and substances that can be excreted from the body.