Critical
ritonavir × Sildenafil
HIV protease inhibitor / pharmacokinetic booster×Phosphodiesterase type 5 inhibitors (PDE5)
Mechanism
Sildenafil is cleared via hepatic CYP3A4. Ritonavir is the strongest CYP3A4 inhibitor available. Sildenafil levels rise 11-fold. The risk of severe hypotension and priapism rises sharply.
Symptoms
Sudden blood pressure drop: dizziness, syncope. Priapism – painful erection lasting more than 4 hours, requiring emergency care. Visual and auditory disturbances are amplified.
Management
For erectile dysfunction on ritonavir, sildenafil is capped at 25 mg no more than once every 48 hours. For pulmonary arterial hypertension (which needs regular high doses), the combination is contraindicated. Alternatives: inhaled iloprost or bosentan with dose adjustment.