Rationale: Current therapies for obstructive sleep apnea (OSA) are limited by insufficient efficacy, compliance, or tolerability. An effective pharmacological treatment for OSA is warranted. Carbonic anhydrase inhibition has been shown to ameliorate OSA. Objectives: To explore safety and tolerability of the carbonic anhydrase inhibitor sulthiame (STM) in OSA. Methods: A 4-week double-blind, randomized, placebo-controlled dose-guiding trial was conducted in patients with moderate and/or severe OSA not tolerating positive airway pressure treatment. Measurements and Main Results: Intermittent paresthesia was reported by 79%, 67%, and 18% of patients receiving 400 mg STM (n = 34), 200 mg STM (n = 12), and placebo (n = 22), respectively. Dyspnea was reported after 400 mg STM (18%). Six patients in the higher dose group withdrew because of adverse events. There were no serious adverse events. STM reduced the apnea-hypopnea index from 55.2 to 33.0 events/h (-41.0%) in the 400-mg group and from 61.1 to 40.6 events/h (-32.1%) after 200 mg (P < 0.001 for both). Corresponding placebo values were 53.9 and 50.9 events/h (-5.4%). The apnea-hypopnea index reduction threshold of ⩾50% was reached in 40% of patients after 400 mg, 25% after 200 mg, and 5% after placebo. Mean overnight oxygen saturation improved by 1.1% after 400 and 200 mg (P < 0.001 and P = 0.034, respectively). Patient-related outcomes were unchanged. Conclusions: STM showed a satisfactory safety profile in moderate and/or severe OSA. STM reduced OSA, on average, by more than 20 events/h, one of the strongest reductions reported in a drug trial in OSA. Larger scale clinical studies of STM in OSA are justified. Clinical trial registered with www.clinicaltrialsregister.eu (2017-004767-13).

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