Summary The aim of the study was to examine the efficacy and safety of temazepam on nocturnal oxygenation and next‐day performance at altitude. A double‐blind, randomized, cross‐over trial was performed in Thirty‐three healthy volunteers. Volunteers took 10 mg of temazepam and placebo in random order on two successive nights soon after arrival at 5000 m, following a 17‐day trek from 410 m. Overnight SaO2 and body movements, and next‐day reaction time, maintenance of wakefulness and cognition were assessed. Compared with placebo, temazepam resulted in a reduction in periodic breathing from a median (range) of 16 (0–81.3)% of the night to 9.4 (0–79.6)% (P = 0.016, Wilcoxon's signed‐rank test), associated with a small but significant decrease in mean nocturnal SaO2 from 78 (65–84)% to 76 (64–83)% (P = 0.013). There was no change in sleep latency (P = 0.40) or restlessness (P = 0.30). Temazepam had no adverse effect on next‐day reaction time [241 (201–380) ms postplacebo and 242 (204–386) ms post‐temazepam], maintenance of wakefulness (seven trekkers failed to maintain 40 min of wakefulness postplacebo, and four post‐temazepam), cognition or acute mountain sickness. At high altitude temazepam reduces periodic breathing during sleep without an adverse effect on next‐day reaction time, maintenance of wakefulness or cognition. The 2% reduction in mean SaO2 post‐temazepam is likely to be predominantly because of acclimatization, as by chance more trekkers took temazepam on the first night (19 versus 14). We conclude that at high altitude temazepam is effective in reducing periodic breathing, and is safe to use, without any adverse effect upon next‐day performance.

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