Definition: CYP3A4 is an enzyme in the body that breaks down many medicines. Drugs and foods that block it (inhibitors) can raise the level of another drug in the blood, and those that speed it up (inducers) can lower it. Labels name CYP3A4 when this changes the dose or safety.
The Viagra label says ritonavir raised sildenafil blood levels about 11-fold, and saquinavir and erythromycin about 3-fold. With ritonavir the label caps the dose at 25 mg in 48 hours.
The Addyi label goes further. Moderate or strong CYP3A4 inhibitors are contraindicated because they can cause severe low blood pressure and fainting. Its examples:
In one study on that label, fluconazole raised flibanserin exposure 7-fold and the study was stopped.
The Addyi label lists rifampin, carbamazepine, phenytoin and St. John's Wort as CYP3A4 inducers that sharply lower flibanserin levels, and says using them together is not recommended. St. John's Wort is sold as a dietary supplement, which is a reminder that interactions are not limited to prescriptions.
An interaction shows up when two things overlap in time. A log with every medicine and supplement, including short antibiotic courses, lets a pharmacist check them together. It can also explain why a familiar dose suddenly feels stronger or weaker, because the clearance of the drug has changed.
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