Drug interactions are typically characterized as either pharmacokinetic or pharmacodynamic. Pharmacokinetic interactions are those in which one drug alters the rate or extent of absorption, distribution, metabolism or excretion of another drug. Pharmacodynamic interactions involve a change in a patient's response to a drug induced by another drug. Pharmacological interactions may be difficult to identify. Concurrent use of two or more drugs with either synergistic or antagonistic effects are examples of this type of interaction.
Drugs can interact at various locations in the body.
The most well-studied site of drug interactions is the liver, which metabolizes many medications. It is responsible for first phase of biotransformation for a variety of drugs. It comprises more than 30 isoenzymes. The most commonly seen isoenzymes involved in drug interactions include CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A4. In addition to looking for pharmacokinetic and pharmacodynamic drug interactions, we must also be aware of the need to consider the pharmacology of the individual agents and how they impact the efficacy of each other.