Concomitant use of sildenafil with ritonavir is contraindicated in pulmonary arterial hypertension patients. Concomitant use of sildenafil with ritonavir dosed as an antiretroviral agent or as a pharmacokinetic enhancer is not recommended and in no instance should sildenafil doses exceed 25 mg in 48 hours. Coadministration of sildenafil (100 mg single dose) and ritonavir (500 mg twice daily) increase sildenafil AUC by 11-fold and Cmin by 4-fold. The increase in sildenafil concentrations may result in associated adverse events such as hypotension and prolonged erection.
Norvir Summary of Product Characteristics, AbbVie Ltd, September 2016.

The effect of ritonavir (500 mg bd) on a single dose of sildenafil (100 mg) was investigated in 28 healthy volunteers. There was an ~11 fold increase in sildenafil AUC and a ~4-fold increase sildenafil Cmax in the presence of ritonavir. Ritonavir delayed the Tmax of sildenafil by ~3.5h and half-life was increased from 3.8 to 5.7 h. Sildenafil did not significantly affect the pharmacokinetics of ritonavir. A sildenafil starting dose of 25 mg should be considered in patients receiving concomitant potent CYP3A4 inhibitors.
Pharmacokinetic interactions between the HIV protease inhibitors ritonavir and saquinavir and Viagra (sildenafil citrate). Muirhead GJ, Wulff MB, Fielding A, et al. Br J Clin Pharmacol, 2000, 50:99–107