Acetazolamide Emerges as the Most Reliable Drug for Acute High Altitude Illness Prevention
Travel to elevations above 2500 metres can trigger a cascade of cerebral and pulmonary stress responses, and the clinical syndromes grouped under high altitude illness range from acute mountain sickness to the far more dangerous high altitude cerebral oedema and high altitude pulmonary oedema. In this Cochrane review, investigators evaluated the most commonly used pharmacological strategies for prevention, bringing together randomized and cross-over trials conducted in mountain settings and in hypobaric chambers that simulate altitude exposure. The question was practical and narrow: which drugs actually reduce illness before ascent, and which ones expose travelers to side effects without enough evidence of benefit?
The clearest answer involved acetazolamide. Across 28 parallel studies involving 2345 participants, and 16 studies with analyzable acute mountain sickness outcomes, the drug reduced the risk of acute mountain sickness compared with placebo, with a risk ratio of 0.47 and no statistical heterogeneity. That is a substantial preventive effect in a condition driven by hypoxia, impaired ventilatory acclimatization, and fluid shifts within the central nervous system. Acetazolamide’s carbonic anhydrase inhibition produces a mild metabolic acidosis that stimulates ventilation, helping travelers adapt more rapidly to the low-oxygen environment. In clinical terms, the mechanism fits the disease.
A single sentence captures the bedside implication: acetazolamide remains the strongest pharmacologic option in this review.
The evidence base for prevention of the rarer but more severe altitude syndromes was thinner. No events of high altitude pulmonary oedema were reported in the acetazolamide trials, and only one high altitude cerebral oedema event appeared across six studies with 1126 participants, leaving those outcomes too sparse for confident estimation. Even so, the data on acute mountain sickness alone give the drug a clear preventive role, and the review’s conclusion supports use in daily doses between 250 and 750 mg. The quality of evidence for this comparison was rated moderate, which matters because the signal comes from a body of studies large enough to support clinical confidence, even as rare endpoints remain underpowered.
Side Effects, Dosing Windows, and the Limits of the Evidence Base
The adverse event profile was less reassuring, though still clinically manageable. Few studies reported side effects in a systematic way, but the available data showed an increased risk of paraesthesia with acetazolamide, with a risk ratio of 5.53 across five studies and 789 participants. For climbers and travelers, that symptom profile is familiar: tingling in the fingers, toes, or face can be bothersome enough to affect adherence, even when the drug is doing exactly what it is designed to do. The evidence for paraesthesia was low quality, yet the direction of effect is consistent with routine clinical experience and the known pharmacology of carbonic anhydrase inhibition.
The timing of prophylaxis was variable across trials. Twenty-four studies started treatment three to five days before ascent, while 23 began one to two days beforehand. Most studies reached a final altitude between 4001 and 5000 metres, which means the findings apply most directly to travelers entering a fairly demanding hypoxic environment rather than to mild altitude exposure. The majority of trials were conducted in high altitude mountain regions, while others used hypobaric chambers. Both settings are relevant, but neither fully reproduces the unpredictability of real-world ascent, where exertion, dehydration, sleep disruption, and rate of climb can compound risk.
Risks of bias were unclear for several domains, and many studies failed to report adverse events in a way that allows full safety assessment. This is not a trivial gap. Prevention trials in altitude medicine often focus on symptom scores and short follow-up windows, leaving clinicians with limited information on less common harms, drug tolerability during longer trips, and the practical consequences of starting treatment before travel. A prophylactic agent can look excellent on paper and still be unusable if its side effects interfere with trekking, work, or sleep.
Budesonide and Dexamethasone Remain Less Certain for Routine Prevention
The review examined six recommended drug classes, but the clinical confidence did not extend evenly across them. Budesonide was supported by only two parallel studies with 132 participants, yet those trials showed a reduction in acute mountain sickness compared with placebo, with a risk ratio of 0.37 and no heterogeneity. The signal is interesting, especially because an inhaled corticosteroid could theoretically reduce airway inflammation and perhaps influence some hypoxia-related responses, but the evidence is still low quality. No events of high altitude pulmonary oedema or cerebral oedema were reported, and no side effects were found in those small studies. The small sample size leaves a large amount of uncertainty around both benefit and harm.
Dexamethasone did not deliver a consistent preventive benefit. Seven parallel studies with 205 participants were pooled, and the data did not show benefit at any dosage, with a risk ratio of 0.60 and a confidence interval reaching 1.00. Evidence quality was low, and adverse event data were rated very low quality. This matters because dexamethasone has long been used in altitude medicine, especially for treatment of established disease or when rapid descent is impossible. Prevention is a different use case. A steroid may blunt symptoms through anti-inflammatory and membrane-stabilizing effects, but routine prophylaxis requires a cleaner tradeoff than this review could confirm.
The review also included other commonly used agents such as ibuprofen, but the evidence did not rise to the level needed for a firm recommendation. Across the broader set of 64 studies and 4547 participants, 26 comparisons were identified, and 15 compared drugs with placebo. Twelve additional studies were ongoing, and another 12 could not be classified because full texts were unavailable. Those missing trials may eventually change the field, but they were not available for the current analysis.
For now, the practical message is anchored in one agent and one outcome: acetazolamide reduces acute mountain sickness, while the case for other commonly used preventive drugs remains uncertain. The review ends where good clinical evidence often does, not with a neat hierarchy of certainty for every medication, but with a clear therapy at the top and several familiar alternatives still waiting for larger, better-reported multicentre studies.