P450 inducers and inhibitors

P450 inducers = increase metabolism of drugs metabolised by cytochrome P450 and thus decrease drug effect

• Carbamazepine 
• Phenytoin 
• Rifampicin 
• Phenobarbital 
• Tobacco 
• Chronic alcohol 
• St Johns Wort 

P450 inhibitors = decrease metabolism of drugs metabolised by cytochrome P450 and thus increase drug effect

• Cardiac
• Amiodarone 
• Verapamil
• Quinidine
• Antibiotics
• Ciprofloxacin
• Erythromycin
• Metronidazole
• Trimethoprim
• Isoniazid
• Antifungals
• Fluconazole
• Terbafine
• HIV antivirals
• Antacids
• Cimetidine
• Omeprazole
• Psychiatric drugs
• TCA
• SSRIs especially fluoxetine
• Haloperidol
• Others
• Methadone
• Grapefruit juice

Drugs metabolised by cytochrome P450 include:

• Cardiac
• Beta blockers
• Angiotensin II blockers
• Statins
• Psychiatric drugs
• TCA
• SSRI
• antipsychotics
• Analgesics
• Tramadol
• Codeine
• Ibuprofen
• Antiepileptics
• Other
• Sildenafil
• Warfarin
• Donepezil
• PPIs
• Glipizide
• Theophylline

Small print gem: the name cytochrome P450 comes from the fact the enzymes are inside the cell (cyto) and contain a haem pigment (chrome P) that absorbs light at a wavelength of 450nm when exposed to carbon monoxide

References:

Kalra, B. Cytochrome P450 enzyme isoforms and their therapeutic implications: an update. Indian j Med Sci. 2007; 61(2): 102-116.

Lynch, T. and Price, A. The effect of cytochrome P450 metabolism on drug response, interactions and adverse events. Am Fam Physician. 2007; 76(3): 391-396.

Hennessy, M. et al. St Johns wort increases expression of P-glycoprotein: implications for drug interactions. Br J Clin Pharmacol. 2002; 53(1): 75-82. 

Indiana University School of Medicine. P450 drug interactions table. Avaliable at: http://medicine.iupui.edu/clinpharm/ddis/table.aspx

Ogu, C. and Maxa, J. Drug interactions due to cytochrome P450. Proc(Bayl Univ Med Cent. 2000; 13(4): 421-423.