WADA adds Carbon Monoxide Rebreathing to 2026 Prohibited List — But not entirely

The World Anti-Doping Agency (WADA) has officially added carbon monoxide (CO) rebreathing to its Prohibited List, categorizing it as a performance-enhancing method under section M1.4 of the 2026 code. However, the move stops short of a full ban — the use of CO for diagnostic purposes will still be permitted.

WADA’s update follows growing scrutiny over the use of carbon monoxide within professional cycling, where a handful of World Tour teams were linked to the practice during the 2024 season. While no evidence of direct performance enhancement was found at the time, the potential for CO to stimulate erythropoiesis — the production of red blood cells — has raised concerns about its manipulation in elite sport.

“The non-diagnostic use of carbon monoxide (CO) was added to the Prohibited Methods as a new section, M1.4,” WADA stated in its announcement. “It can increase erythropoiesis under certain conditions.”

CO inhalation can simulate the effects of altitude training by inducing a state of mild hypoxia — a lack of oxygen — which in turn prompts the body to produce more oxygen-carrying red blood cells. That physiological response is highly sought after in endurance sports like cycling, where oxygen transport can directly influence performance.

Importantly, WADA clarified that diagnostic applications remain permitted, specifically when used to assess total haemoglobin mass or pulmonary diffusion capacity. These tests have been employed by teams to measure blood volume and track physiological adaptations to altitude training, and are considered legitimate medical procedures when done appropriately. “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,” WADA said.

The UCI, had previously urged WADA to take a clear position: “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,” it stated in a 2024 press release.

While the use of CO rebreathers for performance purposes now faces an outright ban from 2026 onward, the nuanced policy approach reflects the complex dual role of CO in both medical diagnostics and potential doping practices. For teams, the challenge will now lie in staying on the right side of that line.