16th May 2025

Enzyme inducers and inhibitors

Cytochrome P450 (CYP450) are a group of enzymes encoded by P450 genes that are responsible for the metabolism of many drugs that are used in clinical practice. Most drugs (approximately 90%) are metabolised by CYP1A2, CYP3A4, CYP3A5, CYP2C9, CYP2C19, CYP2D6, of these CYP3A4 AND CYP2D6 are the most significant.

There are several substrates that bind to and are metabolised by the CYP450 enzymes. Common substrates include:

• Amitriptyline
• Caffeine
• Codeine
• Phenytoin
• Selective serotonin reuptake inhibitors
• Statins
• Theophylline
• Warfarin

There are many medications that can increase the expression (induce) or reduce the rate (inhibit) of CYP450 enzyme catalysed reactions; respectively called enzyme inducers and inhibitors. In the case of enzyme inducers, they will increase the metabolism of substrates resulting in a reduction in circulating plasma levels, the contrary is true of inhibitors. Common enzyme inducers and inhibitors can be found in table 1.