Ritonavir markedly increases the levels of fentanyl, and markedly increases alfentanil- induced miosis. Other
protease inhibitors may also be expected to interact similarly but there seems little documented about these effects in practice. Care should also be taken if
ritonavir is used with other
protease inhibitors as a pharmacokinetic enhancer.
Concurrent use should be monitored. Dose reductions of
fentanyl (given by any route) or
alfentanil may be required, particularly with long-term use.