No pharmaceutical opinion available for this interaction.
Ritonavir may possibly inhibit the metabolism (CYP 3A4) and consequently increase the plasma concentration of Tadalafil (Cialis).
3A4 and 2D6 (major), 1A2 and 2B6 (minor), 5 metabolites, one of which has a weak concentration of an active metabolite (M-2), transporters: MRP1, MRP2, P-gp
2B6, 2C8, 2C9, 1A2, 2C19 (moderate) UGT (moderate)
3A4 (potent) and 2D6 (moderate), transporters: BCRP, OATP1A2, OATP1B1, OATP1B3, MRP1, MRP2, OCT1, OCT2, P-gp
Possible increase of adverse effects.
Use as needed: start at 10 mg every two days and if needed increase to 20 mg, maximum of 3 times a week.
If the 10 mg dose is not tolerated, tadalafil treatment should be stopped.
Once-daily use: usual dose of 5 mg QD could be used. If intolerance, reduce the dose to 2.5 mg QD.
Monitor closely clinical efficacy and appearance of adverse effects.
Give extra caution to patients with hepatic or renal insufficiency, or older than 65 years.
Tadalafil toxicity: hypotension, tachycardia, headache, dizziness, flushing, visual changes (difficulty distinguishing blue and green) and syncope.
Ritonavir 200 mg BID (a powerful inhibitor of CYP 3A4) increases the AUC of tadalafil 124%.
Caution: Liver impairment and kidney failure (Clcr 31-50 ml/min: increases twice AUC)and age > 65 years old can increase the concentration of tadalafil. See tadalafil (Adcirca).