Ranitidine (Zantac) is a histamine H₂-blocker, which inhibits stomach acid production. It is used to treat stomach ulcers and acid reflux disease (heartburn). It is marketed by GlaxoSmithKline.

The commonest side-effect of ranitidine is headache.

The absorption of some protease inhibitors, notably atazanavir (Reyataz) is dependent upon the stomach acid level being high enough. Since ranitidine lowers stomach acid levels, there is a risk that patients taking atazanavir may have low levels of the protease inhibitor in the blood, putting them at risk of drug resistance and treatment failure^[1]. Consequently, atazanavir’s manufacturer recommends that ranitidine be taken twelve hours apart from atazanavir. A similar interaction may occur with fosamprenavir (Telzir): ranitidine should be used cautiously by patients taking this protease inhibitor^[2]. Patients taking tipranavir (Aptivus) should also use ranitidine with care, until the interaction between these drugs has been studied.

Ranitidine does not have significant interactions with any other antiretroviral drugs^[3]. However, ddI (didanosine, Videx) tablets should be taken at least two hours after ranitidine, as both drugs lower stomach acid levels^[4].

Despite one study suggesting that ranitidine could enhance the activity of natural killer cells in people with HIV, a larger, randomised, double-blind study showed that ranitidine had no impact on CD4 cell counts or viral load during early HIV infection^[5][6].