A once-daily pill called sulthiame cut breathing interruptions during sleep by up to 47 percent in adults with obstructive sleep apnea, according to a completed phase 2 clinical trial conducted across 28 sites in five European countries. The FLOW trial, which randomized 298 patients over 15 weeks, represents the largest controlled test yet of a drug-based alternative for a condition that affects tens of millions of people worldwide, many of whom abandon standard treatments like CPAP machines. The results, published in The Lancet, add weight to a growing body of evidence that a simple tablet taken before bed could reshape how moderate-to-severe sleep apnea is managed.
What the FLOW Trial Found
The FLOW trial was a multicentre, randomised, double-blind, placebo-controlled, dose-finding study. Researchers screened 535 patients between December 2021 and April 2023, ultimately randomizing 298 adults into four groups: placebo (75 patients) or one of three daily doses of sulthiame at 100 mg, 200 mg, or 300 mg. The primary measure was the apnea-hypopnea index, or AHI, which counts how many times per hour a person’s breathing partially or fully stops during sleep.
The results showed a clear dose-dependent pattern. At the lowest dose of 100 mg, the placebo-adjusted change in AHI at week 15 was a relative reduction of 16.4 percent. Higher doses produced steeper reductions, with patients on the largest doses experiencing up to 47 percent fewer breathing pauses compared to those taking a placebo. That dose–response curve is significant because it suggests the drug’s effect is pharmacologically specific rather than a statistical fluke, and it gives future trials a clearer target for optimal dosing.
Beyond the primary endpoint, the trial also evaluated secondary measures such as oxygen desaturation and time spent in deeper stages of sleep. Although full details are still being parsed, early reporting indicates that improvements in breathing translated into better overnight oxygen levels, a critical factor for protecting the heart and brain from chronic intermittent hypoxia. Importantly, these benefits were observed in a population with moderate-to-severe disease, the group at highest risk for long-term complications.
Earlier Evidence Set the Stage
The FLOW results did not emerge from a vacuum. A smaller randomized, placebo-controlled, double-blind study published in 2022 in the American Journal of Respiratory and Critical Care Medicine had already tested sulthiame in patients with moderate-to-severe obstructive sleep apnea who were either intolerant of or unwilling to use CPAP. In that four-week trial, investigators reported that 40 percent of participants receiving 400 mg of sulthiame achieved what researchers defined as clinically relevant alleviation: a reduction in AHI of at least 50 percent.
By comparison, only a small fraction of patients on placebo reached that threshold, and those given a 200 mg dose had an intermediate response. A companion analysis on dose-specific responder rates found that about a quarter of patients at 200 mg met the same 50 percent reduction benchmark, reinforcing the idea that higher doses yield stronger effects but may also carry more side effects.
That earlier trial was short and relatively small, but it established two things. First, sulthiame could produce meaningful reductions in breathing interruptions within weeks. Second, the effect was dose-dependent even at that stage. The FLOW trial extended the observation window to 15 weeks and tested lower doses across a much larger patient population, confirming the pattern at scale and providing a more nuanced picture of how different dose levels balance efficacy and tolerability.
Why a Pill Matters for Sleep Apnea Patients
Obstructive sleep apnea is typically treated with CPAP, a device that pushes pressurized air through a mask worn during sleep. CPAP works well when used consistently, but adherence is notoriously poor. Many patients find the mask uncomfortable, disruptive to sleep partners, or impractical for travel. Others simply refuse the device after an initial trial. The 2022 sulthiame study specifically enrolled patients who fell into these categories, a design choice that signals where researchers see the biggest unmet need.
A pill taken once before bed sidesteps most of those compliance barriers. Sulthiame, originally developed decades ago as an anti-epileptic drug, acts on carbonic anhydrase enzymes in ways that appear to stabilize respiratory drive during sleep. A separate biomarker-focused analysis tied to the sulthiame clinical trial program has explored what the drug changes beyond AHI, including potential effects on ventilatory control and markers of nocturnal hypoxia. That mechanistic work matters because AHI alone does not capture the full burden of sleep apnea, which includes daytime sleepiness, cardiovascular strain, and metabolic disruption.
The practical appeal is straightforward: if a once-daily tablet can halve the number of breathing pauses per hour, patients who currently go untreated because they cannot tolerate CPAP would have a viable alternative. That population is large, and the downstream health consequences of untreated sleep apnea, including elevated risk of hypertension, stroke, arrhythmias, and motor vehicle accidents, are well documented. Even for patients who do use CPAP, a drug that partially reduces AHI might allow lower pressure settings or more flexible use, potentially improving comfort and adherence.
What the Trial Did Not Answer
For all its promise, the FLOW trial leaves several questions open. Fifteen weeks is long enough to establish short-term efficacy but far too brief to assess whether the drug’s benefits hold over months or years of nightly use. Sleep apnea is a chronic condition, and any treatment must prove durable. The public trial record confirms that the study has reached completion, but no data yet address whether patients maintain their response over extended periods, or whether some eventually drift back toward baseline AHI despite continued dosing.
Side effects also warrant close attention. Coverage of the trial notes that adverse events in the treatment arms were generally mild and temporary, but the specific rates and types of side effects at each dose level will shape whether regulators and clinicians view the drug’s risk–benefit profile favorably. Sulthiame’s known side-effect profile from its use in epilepsy includes tingling sensations, changes in taste, and gastrointestinal symptoms, all of which can erode adherence if they are frequent or persistent. Because obstructive sleep apnea is not immediately life-threatening for most patients, tolerance for side effects is typically lower than in conditions like refractory seizures.
Another unknown is how sulthiame will compare with existing non-CPAP options. Oral appliances that reposition the jaw, positional therapy devices that discourage back-sleeping, and upper-airway surgeries all compete in the same therapeutic space. In moderate-to-severe disease, these alternatives often fail to normalize AHI, but they can still provide partial relief. Whether sulthiame will outperform those approaches, or be used alongside them in combination regimens, will depend on forthcoming head-to-head or pragmatic effectiveness studies.
There are also important questions about which patients stand to benefit most. Obstructive sleep apnea is a heterogeneous disorder; some patients have collapsible upper airways as the dominant problem, while others have unstable ventilatory control or low arousal thresholds. The biomarker work already underway suggests that certain physiological traits may predict a stronger response to carbonic anhydrase inhibition. Identifying those subgroups could help clinicians avoid a trial-and-error approach and reserve sulthiame for patients most likely to see meaningful gains.
What Comes Next
Regulatory pathways for a repurposed drug like sulthiame are complex. Although the compound has a long history in neurology, its safety and dosing profile in a predominantly middle-aged, often cardiometabolic population with sleep apnea must be evaluated independently. The FLOW trial provides a strong efficacy signal and an encouraging safety snapshot, but larger and longer studies will be needed before agencies can fully assess the balance of benefits and risks.
According to the detailed registration entry, investigators designed FLOW as a phase 2 dose-finding study, which typically precedes one or more pivotal phase 3 trials. Those future studies will likely focus on the dose or doses that showed the best combination of AHI reduction and tolerability, while incorporating more robust assessments of daytime functioning, quality of life, cardiovascular biomarkers, and real-world adherence.
If those trials confirm the early promise, sulthiame could become the first broadly available oral therapy specifically approved for obstructive sleep apnea. That would not replace CPAP, which remains the gold standard for eliminating apneas in many patients, but it would add an important option to a toolkit that has changed little in decades. For the large share of people who cannot or will not sleep every night with a mask and hose, a pill that meaningfully reduces breathing interruptions could mark the difference between living for years with an under-treated condition and finally getting a manageable, sustainable therapy.
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*This article was researched with the help of AI, with human editors creating the final content.
