St John's wort is predicted to reduce the levels of
phosphodiesterase type-5 inhibitors, because other inducers of CYP3A4 have been shown to do so. For example, the AUC of sildenafil is reduced by about 70% by bosentan and the AUC of
tadalafil is reduced by 88% by rifampicin. Vardenafil is also metabolised by CYP3A4, and therefore its levels may possibly be lowered by St John's wort.
If these
phosphodiesterase type-5 inhibitors are not effective in patients taking St John's wort, consider reviewing the need for concurrent use. If concurrent use is necessary, a dose increase in the
phosphodiesterase type-5 inhibitor may be required.