PHENYTOIN AND PHOSPHODIESTERASE TYPE-5 INHIBITORS
Phenytoin 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 phenytoin.
If these phosphodiesterase type-5 inhibitors are not effective in patients taking
phenytoin, it would seem sensible to try a higher dose with close monitoring. Note
that fosphenytoin, a prodrug of phenytoin, may interact similarly.
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 phenytoin.
If these phosphodiesterase type-5 inhibitors are not effective in patients taking
phenytoin, it would seem sensible to try a higher dose with close monitoring. Note
that fosphenytoin, a prodrug of phenytoin, may interact similarly.