NEW YORK, July 31 (Praxis Press) Several medications have been used to alleviate or prevent acute mountain sickness (AMS), but their relative efficacies are unknown. Dumont and colleagues reviewed 33 randomized placebo-controlled trials of drugs used to prevent AMS. The mean incidence of AMS above 4,000 meters was 67% in subjects taking a placebo; the rate of ascent correlated with incidence, whereas altitude and mode of ascent did not. Dexamethasone (8-16 mg) and acetazolamide (750 mg) prevented AMS (relative risks [RR], 2.50 and 2.18, respectively; number needed to treat, 2.8 and 2.9, respectively). Lower doses of dexamethasone (0.5 or 2 mg) and acetazolamide (500 mg) were ineffective. Dexamethasone was associated with adverse effects upon abrupt discontinuation (RR, 4.45); acetazolamide was associated with parasthesias (RR, 4.02) and polyuria (RR, 4.24). At ascent rates greater than 500 meters/day above 4,000 meters, higher doses of dexamethasone and acetazolamide are equally effective for preventing AMS.