A year after controversy erupted over the use of carbon monoxide rebreathers by some of professional cycling's top teams, the World Anti-Doping Agency (WADA) has added the use of the gas as a performance-enhancing method to its Prohibited List, starting in 2026.
"The non-diagnostic use of carbon monoxide (CO) was added to the Prohibited Methods as a new section, M 1.4," WADA stated in its summary of the major modifications to the Prohibited List. "It can increase erythropoiesis under certain conditions."
The controversy began with a report by the Escape Collective website during the 2024 Tour de France, which stated that UAE Team Emirates, Visma-Lease a Bike, and Israel-Premier Tech used carbon monoxide as a training measurement method. There was no evidence that the teams had used carbon monoxide specifically for performance-enhancing purposes.
The gas, which is deadly when inhaled in sufficient doses, can simulate the type of hypoxia one would get when at high altitudes. Hypoxia, a lack of oxygen, stimulates the body to produce more oxygen-carrying red blood cells, and altitude camps have become the standard for professional cyclists wanting to achieve peak performance.
Carbon monoxide can also be used as a diagnostic method, which the 2026 WADA code will still allow. Teams stated they used the gas to measure blood volume.
"I understand if it's misused, but I never knew that it could be misused," Visma's Jonas Vingegaard said at the team's press conference last November. "I think I said before that we only used it to test if the altitude camps are working or not."
The UCI then moved to request that WADA explicitly ban carbon monoxide rebreathers as a performance enhancement.
"The UCI clearly asks teams and riders not to use repeated CO inhalation. Only the medical use of a single inhalation of CO in a controlled medical environment could be acceptable," the UCI press release stated last year.
"The UCI is also officially requesting the World Anti-Doping Agency (WADA) to take a position on the use of this method by athletes."
The Escape Collective report concluded that there was "no hard evidence that any WorldTeams are currently using CO inhalation for performance gains", but the suspicion that the story raised led UAE Team Emirates to announce they would cease using the gas as a diagnostic method in December.
The recent WADA explanation makes clear that using the gas as a diagnostic measure is still allowed.
"Carbon monoxide for diagnostic purposes, such as total haemoglobin mass measurements or the determination of pulmonary diffusion capacity, is not prohibited. The current wording was chosen to differentiate between illicit use and the intake resulting from natural combustion processes (e.g. smoking), the environment (e. g. exhaust gases) or diagnostic procedures."
