Key Principles of Pharmacology

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2. Pharmacokinetics

2.3. Metabolism (medicine biotransformation)

Metabolism is the conversion of one chemical compound into another. Most metabolism of medicines occurs in the liver in the ‘first-pass effect’, although some processes occur in the gut wall, lungs and blood plasma. On the whole, as medicines are metabolised their therapeutic effect changes.

Substances (medicines, toxins, foods, etc.) absorbed into the bloodstream from the stomach and intestinal tract, enter the liver via the hepatic portal vein before entering the general circulation. Medicines or other substances are metabolised or inactivated in the liver before they are distributed throughout the body or eliminated by the kidneys.

In some instances it is not the medicine itself which exerts an effect but one or more of its so called ‘active metabolites’ (any substance produced during metabolism). An active metabolite results when a medicine is metabolised 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 (e.g. morphine-6-glucuronide). Certain drugs such as codeine and tramadol have metabolites 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 medicine. Sometimes, however, metabolites may produce toxic effects and patients must be monitored carefully to ensure the toxins do not build up in the body.