Drug Metabolism:
Drug metabolism is the term which describes the biotransformation of pharmaceutical/illicit substances in the body so that they can be eliminated more easily. The majority of metabolic processes that involve drugs occur in the liver, where the enzymes which facilitate these reactions are most concentrated.

The purpose of metabolism in the body is usually to change the chemical structure of the substance, to increase the ease with which it can be excreted from the body.
Drugs are metabolized through various reactions including:

• Oxidation
• Reduction
• Hydrolysis
• Hydration
• Conjugation
• Condensation
• Isomerization

In most cases, when a drug is metabolized it becomes inactivated. However, the metabolites of some drugs are pharmacologically active and exert an effect on the body. In fact, the active metabolite of some medications is responsible for the principal action of the drug. In this case, the drug formulation is referred to as a prodrug.

The rate of drug metabolism varies significantly for different patients. This affects the efficacy and toxicity of the drug for different patients who have high or low metabolism rates.
For example, those with fast metabolisms clear the drug very quickly, and the therapeutic concentration of the drug in the blood and tissues may not be reached. In other patients, the drug is metabolized so slowly that it accumulates in the bloodstream. The higher concentration of the drug in the body creates a greater potential for adverse effects.

The patient factors that affect the rate of metabolism include:

• genetic predisposition
• chronic liver disorders
• advanced heart failure
• interactions with other concurrent medications/substances
• ageing (the capacity of the CYP450 metabolism decreases by at least 30% with age)

Drug interaction is a term referring to the metabolism of some drugs being affected by the presence of other substances. During metabolism, enzymes act as catalysts for the oxidation of many drugs. They can in turn, also be induced or inhibited by many drugs and other substances. This is what's known as drug interactions.