Ritonavir (Norvir) is able to reduce HIV viral load and increase CD4 cell counts in the majority of people taking it in combination with at least two other antiretroviral drugs.

Ritonavir was approved after two main studies demonstrating its effectiveness in reducing AIDS-defining events and viral loads when added to an existing nucleoside reverse transcriptase inhibitor (NRTI) regimen (Study 247), and in reducing viral loads in treatment-naive patients (Study 245)^[1]. Interestingly, the latter study showed equivalent effectiveness in patients treated with ritonavir plus AZT (zidovudine, Retrovir) and those treated with ritonavir alone, over 48 weeks. This was probably due to side-effects and poor adherence in the dual therapy arm.

Subsequent studies found that ritonavir-based HIV treatment was as effective as treatment based on indinavir (Crixivan) in patients with prior exposure to NRTIs. After over a year of treatment, similar numbers of patients taking each drug developed a new AIDS-defining condition or died, and CD4 cell count increases were similar^[2].

Ritonavir does not cross the blood-brain barrier as well as NRTIs such as AZT or d4T (stavudine, Zerit)^[3]. It also penetrates poorly into the semen^[4].

There is some evidence that ritonavir also has anti-malarial activity, which may be of relevance in areas with high prevalence of HIV and malaria^[5].