Carbamazepine is predicted to reduce the levels of
phosphodiesterase type-5 inhibitors, because other inducers of CYP3A4 have been shown to do so. For example, sildenafil levels are reduced by 70% by bosentan and
tadalafil levels are reduced by 88% by rifampicin. Vardenafil is also metabolised by CYP3A4, and therefore its levels may possibly be lowered by
carbamazepine.
If these
phosphodiesterase type-5 inhibitors are not effective in patients taking
carbamazepine, it would seem sensible to try a higher dose with close monitoring.