Understanding CYP Enzymes and Drug Interactions

Contributor: Jorge Chalit-Hernandez, OMS3

Educational Pearls:

• CYP enzymes are responsible for the metabolism of many medications, drugs, and other substances

• CYP3A4 is responsible for the majority

• Other common ones include CYP2D6 (antidepressants), CYP2E1 (alcohol), and CYP1A2 (cigarettes)

• CYP inducers lead to reduced concentrations of a particular medication

• CYP inhibitors effectively increase concentrations of certain medications in the body

• Examples of CYP inducers

• Phenobarbital

• Rifampin

• Cigarettes

• St. John's Wort

• Examples of CYP inhibitors

• -azole antifungals like itraconazole and ketoconazole

• Bactrim (trimethoprim-sulfamethoxazole)

• Ritonavir (found in Paxlovid)

• Grapefruit juice

• Clinical relevance

• Drug-drug interactions happen frequently and often go unrecognized or underrecognized in patients with significant polypharmacy

• A study conducted on patients receiving Bactrim and other antibiotics found increased rates of anticoagulation in patients receiving Bactrim

• Currently, Paxlovid is prescribed to patients with COVID-19, many of whom have multiple comorbidities and are on multiple medications

• Paxlovid contains ritonavir, a powerful CYP inhibitor that can increase concentrations of many other medications

• A complete list of clinically relevant CYP inhibitors can be found on the FDA website:

• https://www.fda.gov/drugs/drug-interactions-labeling/drug-development-and-drug-interactions-table-substrates-inhibitors-and-inducers

References

1. Glasheen JJ, Fugit RV, Prochazka AV. The risk of overanticoagulation with antibiotic use in outpatients on stable warfarin regimens. J Gen Intern Med. 2005;20(7):653-656. doi:10.1111/j.1525-1497.2005.0136.x

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

3. PAXLOVID™. Drug interactions. PAXLOVIDHCP. Accessed March 16, 2025. https://www.paxlovidhcp.com/drug-interactions

Summarized & Edited by Jorge Chalit, OMS3

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