Sleep Apnea Pill Cuts Breathing Pauses by 47 Percent

A Repurposed Drug Redefines Sleep Medicine

For the roughly one billion people worldwide who suffer from obstructive sleep apnea (OSA), treatment has long meant strapping on a bulky mask every night and enduring the hum of a continuous positive airway pressure (CPAP) machine. Now, a landmark European clinical trial suggests a nightly pill could achieve similar results — and the compound in question has been around for decades.

The FLOW trial, a multicentre, randomised, double-blind, placebo-controlled Phase 2 study published in The Lancet in October 2025, found that sulthiame — an oral carbonic anhydrase inhibitor long used to treat a form of childhood epilepsy — reduced breathing interruptions in adults with moderate-to-severe OSA by up to 47 percent compared with placebo.

What the Trial Found

The study enrolled 298 adults across five European countries, randomising them to one of three once-daily doses of sulthiame (100 mg, 200 mg, or 300 mg) or placebo for 15 weeks. The primary measure was the apnea-hypopnea index (AHI) — the number of breathing interruptions per hour of sleep — assessed using centrally scored polysomnography, the gold-standard sleep study.

Results showed a clear, dose-dependent improvement: the 200 mg and 300 mg groups reduced OSA severity by approximately 30 to 50 percent. Patients also showed better overnight blood oxygen levels, improved sleep quality, and reduced daytime sleepiness. Researchers described the 200 mg dose as offering the most favourable benefit-to-risk profile.

Side effects were predominantly mild to moderate and resolved over time, with no clinically relevant safety concerns identified in the trial population.

How Sulthiame Works

Obstructive sleep apnea occurs when the upper airway repeatedly collapses during sleep, momentarily cutting off breathing. Unlike CPAP, which physically holds the airway open with pressurised air, sulthiame targets the underlying neuromuscular control of breathing.

As a carbonic anhydrase inhibitor, the drug increases the body's ventilatory drive — essentially making the brain's respiratory control centres more sensitive to CO₂ build-up. This heightened drive stiffens the upper airway muscles and reduces the likelihood of collapse. The same mechanism explains why the drug was originally effective in certain epileptic syndromes involving respiratory dysregulation.

A Potential Breakthrough for Millions

Jan Hedner, senior professor of pulmonary medicine at the Sahlgrenska Academy, University of Gothenburg, and a lead investigator in the trial, called the results transformative.

"We have been working on this treatment strategy for a long time, and the results show that sleep apnea can indeed be influenced pharmacologically. It feels like a breakthrough."

The significance of the finding is hard to overstate. An estimated 425 million middle-aged adults globally meet diagnostic criteria for moderate or severe OSA, yet the condition remains massively undertreated. CPAP is highly effective but notoriously difficult to tolerate: studies suggest that up to half of patients abandon the device within a year due to discomfort, noise, or the social awkwardness of wearing a mask to bed.

A once-daily pill could transform adherence — and reach patients in lower-income settings where CPAP devices are unaffordable or unavailable.

Phase 3 Still Required

Experts and the trial's authors are cautious about declaring victory too soon. The FLOW study was a Phase 2 dose-finding trial, meaning it was designed primarily to establish efficacy signals and determine the right dose — not to provide the definitive evidence needed for regulatory approval. Larger Phase 3 trials with longer follow-up periods will be necessary to confirm that the effect is sustained, identify which patient subgroups benefit most, and fully characterise long-term safety.

The biopharmaceutical company Apnimed, which is developing sulthiame for OSA, has indicated it plans to advance the compound into Phase 3 studies based on these results. If those trials succeed, sulthiame could become the first approved pharmacological treatment for obstructive sleep apnea — a milestone in a field where patients have waited decades for options beyond the mask.