Cimetidine (Dyspamet / Tagamet) is a drug designed to treat ulcers, but it has also demonstrated some potential to boost the immune system. It is a licensed anti-histamine drug that reduces the levels of hydrochloric acid in the stomach. Its use as an anti-HIV immune modulating treatment is experimental.

Histamine is a substance produced by white blood cells in the stomach as a regulator of acid secretion, or in response to an allergic reaction. However, some immune cells also have receptors for histamine: cimetidine is able to attach to these, theoretically blocking the ill effects that histamine may have on the activity of the immune system.

Three published studies have focused on cimetidine as a treatment for HIV infection. The first, an uncontrolled trial in 33 people with AIDS, showed increases in CD4 cell counts and improvements in symptoms such as night sweats and diarrhoea and in skin tests used to measure the extent of immune functioning^[1]. Another study of eight patients with Kaposi’s sarcoma showed some benefit^[2]. However, a placebo-controlled trial of cimetidine found no evidence of an effect on CD4 cell counts or p24 antigen levels^[3].

Cimetidine reverses the normal acidity of the stomach which can affect how certain other drugs are absorbed. Drugs that require an acidic stomach, such as some protease inhibitors, dapsone and ketoconazole (Nizoral), may not be absorbed adequately. In contrast, cimetidine may increase the absorption of ddI (didanosine, Videx / VidexEC), increasing the risk of side-effects such as pancreatitis.

Cimetidine also inhibits the cytochrome P450 system, potentially increasing the levels of some anti-HIV drugs in the blood^[4].

No side-effects were reported in the published trials of cimetidine for HIV infection, but mild diarrhoea, dizziness, rashes, confusion, headaches and reversible liver damage have been reported by some people using cimetidine as an anti-ulcer treatment.

Doctors are able to prescribe cimetidine for conditions where reduced stomach acidity is required. While they are not supposed to prescribe a drug for indications other than those for which it is licensed, some doctors may be willing to write prescriptions for people who wish to try the drugs as immune modulators.

Cimetidine is not available over the counter and is not currently being investigated in clinical trials in the United Kingdom.