Oxybutynin Itraconazole

Itraconazole and ketoconazole double the serum levels of oxybutynin. The clinical relevance of this is uncertain. 

A single 5-mg dose of oxybutynin was given to 10 healthy subjects after they had taken itraconazole 200 mg daily or placebo for 4 days. The peak serum levels and the AUC ofthe oxybutynin were approximately doubled, while the pharmacokinetics of the active metabolite of oxybutynin were... 

This interaction is almost certainly due to itraconazole and ketoconazole inhibiting the metabolism of oxybutynin by the cytochrome P450 isoenzyme CYP3A4, in the intestinal wall and liver. 

The authors of this report consider that this interaction is only of minor importance, but note that this was only a single-dose study and may not necessarily reflect the full picture in practice. Nevertheless, because itraconazole is a known and potent enzyme inhibitor, they prediet that other CYP3A4 inhibitors that are less potent (they name erythromycin, diltiazem, and verapamil) are unlikely to interact with oxybutynin significantly. Nevertheless, some manufacturers recommend eaution with concurrent use, and until more is known this seem prudent. Bear in mind the possibility of an interaction if antimuscarinic effects (dry mouth, constipation, drowsiness) are increased. 

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