Ketoconazole raises the AUC of
tolterodine by more than 2-fold in some patients. Other potent inhibitors of CYP3A4, such as the
protease inhibitors, are predicted to interact similarly.
The UK manufacturers advise avoiding concurrent use. The US manufacturers suggest reducing the
tolterodine dose to 1 mg twice daily, which seems practical. It may be prudent to assess experience of adverse effects in these patients, and to reduce the dose further or withdraw the drug if it is not tolerated.