Common side-effects of ritonavir (Norvir) include headache, nausea, vomiting, diarrhoea, dizziness, and tingling or numbness around the mouth. These are most likely to occur during the first weeks of treatment and there is evidence that women are more likely to suffer from these side-effects than men^[1][2]. Medicines to control nausea, diarrhoea and headache can be prescribed before starting ritonavir.

As a class, protease inhibitors have been associated with a syndrome of fat and metabolic irregularities. This syndrome includes altered body fat distribution, high fat levels in the blood, diabetes, increased levels of blood sugar and increased bleeding in haemophiliacs. Ritonavir has been associated with a more rapid and severe form of body fat changes than other protease inhibitors in some studies. It is also particularly associated with elevated in blood lipids such as cholesterol and triglycerides, although genetic variation in two apolipoprotein genes determines the risk of triglyceride elevations, at least in part^[3]. Combining ritonavir with another protease inhibitors may produce greater increases in triglyceride levels than a single protease inhibitor^[4][5]. For more details, see Body fat and metabolic changes whilst on treatment.

Kidney failure is a rare, serious side-effect of ritonavir^[6]. It is more likely in people with pre-existing kidney problems or who are taking other drugs that are toxic to the kidneys.

While liver toxicity is a side-effect of full-dose ritonavir, it is not associated with the low doses used to boost the levels of other protease inhibitors, even in patients with hepatitis C co-infection^[7].

Low-dose ritonavir may also cause elevations of uric acid in the blood, which may lead to gout, particularly in patients with other risk factors^[8].