★★☆Single study — solid evidence, such as one good trial or a large observational dataset, but not replicated at the top tier.
With ritonavir, one 25mg tablet every 48 hours is the limit
Sildenafil is cleared by a liver enzyme that the HIV medicine ritonavir and the antifungal ketoconazole block, leaving much more of it in the blood for longer.
What the study found
On ritonavir the ceiling is one 25mg tablet in 48 hours
Sildenafil is cleared by a liver enzyme that several common drugs block, among them the HIV medicine ritonavir and the antifungal ketoconazole. Blocking it leaves much more sildenafil in the blood for longer, which is why the label caps ritonavir users at one 25mg dose per 48 hours.
The caveat · No record of anyone past the cap
This is a dosing rule read off a label, not a measured harm rate: the label sets the ceiling without publishing what happened to anyone who exceeded it. The three drugs named are examples of a wider group that blocks the same enzyme.