Coadministration of tadalafil (20 mg single dose) and ritonavir (200 mg twice daily) increased tadalafil AUC by 124%, but had no effect on Cmin. When tadalafil is used concurrently with ritonavir in patients with pulmonary arterial hypertension, refer to the tadalafil SmPC or prescribing information.
Norvir Summary of Product Characteristics, AbbVie Ltd, September 2016.

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. Dose adjustments are recommended for use of tadalafil (Adcirca) for pulmonary arterial hypotension with ritonavir. In patients receiving ritonavir for at least one week, start tadalafil at 20 mg once daily. Increase to 40 mg once daily based upon individual tolerability. In patients on tadalafil, avoid use of tadalafil during the initiation of ritonavir. Stop tadalafil at least 24 hours prior to starting ritonavir. After at least one week following the initiation of ritonavir, resume tadalafil at 20 mg once daily. Increase to 40 mg once daily based upon individual tolerability.
Norvir Prescribing Information, AbbVie Inc, December 2016.