Coadministration of tadalafil (20 mg single dose) and ritonavir (200 mg twice daily) resulted in a 124% increase in tadalafil AUC. Particular caution should be used when prescribing tadalafil in patients receiving ritonavir. Coadministration of ritonavir with tadalafil may result in an increase in tadalafil-associated adverse events, including hypotension, syncope, visual changes and prolonged erection. When using tadalafil for erectile dysfunction, it is recommended not to exceed a tadalafil dose of 10 mg every 72 h and to use with increased monitoring for adverse events.
Norvir Prescribing Information, AbbVie Inc, December 2016.