Background High altitude illness (HAI) is a term used to describe a group of cerebral and pulmonary syndromes that can occur during travel to elevations above 2500 metres (˜ 8200 feet). Acute hypoxia, acute mountain sickness (AMS), high altitude cerebral oedema (HACE) and high altitude pulmonary oedema (HAPE) are reported as potential medical problems associated with high altitude. In this review, the first in a series of three about preventive strategies for HAI, we assess the e ectiveness of six of the most recommended classes of pharmacological interventions. Objectives To assess the clinical e ectiveness and adverse events of commonly-used pharmacological interventions for preventing acute HAI. Search methods We searched the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE (OVID), Embase (OVID), LILACS and trial registries in January 2017. We adapted the MEDLINE strategy for searching the other databases. We used a combination of thesaurus-based and freetext terms to search. Selection criteria We included randomized-controlled and cross-over trials conducted in any setting where commonly-used classes of drugs were used to prevent acute HAI. Data collection and analysis We used standard methodological procedures as expected by Cochrane.