The World Anti-Doping Agency (WADA) has confirmed that carbon monoxide inhalation will be added to its list of banned methods from 2026, closing a controversial loophole that emerged in professional cycling and could have wider implications for endurance sports like running.
Carbon monoxide (CO) is usually associated with car exhausts and faulty heaters, but in recent years, some WorldTour cycling teams began experimenting with CO rebreathers, devices that administer controlled doses of the gas.
The idea is to mimic altitude training: inhaling CO creates hypoxia (oxygen deprivation), which can stimulate the body to produce more red blood cells. More red blood cells means greater oxygen delivery to the muscles, a clear performance boost for athletes competing in aerobic sports.
The practice first came to light during the 2024 Tour de France through reporting by Escape Collective, which revealed that UAE Team Emirates, Visma-Lease a Bike, and Israel–Premier Tech had used CO devices, albeit claiming only for diagnostic purposes.
Teams said they employed the tool to measure blood volume and monitor the effects of altitude camps, not to gain an edge in competition.
Jonas Vingegaard of Visma insisted at a press conference, “We measure the day we get to altitude and then the day we go back down. We see the difference in how much haemoglobin is built up. There is nothing more to it.”
Still, the revelation raised alarm. The Union Cycliste Internationale (UCI) acted first, banning the method in early 2025 and insisting that only single inhalations in controlled medical environments could be justified.
WADA has now followed suit, writing into its 2026 code that “the non-diagnostic use of carbon monoxide” is prohibited, placing it under the section of methods involving manipulation of blood. The agency noted that under certain conditions, CO “can increase erythropoiesis”, or the production of red blood cells.
Importantly, diagnostic use remains permitted. That means CO can still be used to measure total haemoglobin mass or assess lung function under medical supervision, but any attempt to harness it for training or competition is considered doping. Some teams, such as UAE, have already dropped the practice entirely since the initial controversy.
For runners, the ruling might sound like it belongs to another sport. But it is not far-fetched to see why WADA acted: endurance disciplines often share the same marginal-gains culture, from altitude tents to heat chambers to ketone supplements.
Had CO rebreathers remained in the grey zone, they could easily have found their way into running camps, marketed as a shortcut to the red blood cell gains of altitude without the travel. WADA’s ban makes clear that line will not be crossed.
The new Prohibited List also includes other changes, including new stimulant classifications, tighter rules around asthma medications, and expanded definitions of gene and cell doping, but the move against carbon monoxide is the most eye-catching. For now, the message is straightforward: from 2026, inhaling CO to boost performance will be treated as doping.
