Over a year since controversy around carbon monoxide inhalation in cycling began, the World Anti Doping Authority (WADA), has announced it will ban the use of the gas for performance enhancing methods from 2026.
Carbon monoxide (CO) inhalation involves breathing in a dose of the gas through a measurement tool called a CO rebreather. The exposure to the gas can trigger hypoxia (a lack of oxygen), which can lead to the release of red blood cells in the body, improving aerobic capacity like VO2max and maximal oxygen uptake. This form of blood manipulation as a tool to enhance athletic performance is to be banned.
“The non-diagnostic use of carbon monoxide (CO) was added to the Prohibited Methods as a new section, M 1.4,” WADA wrote in an update to its Prohibited List. “It can increase erythropoiesis [the overproduction of red blood cells] under certain conditions.”
Whilst no WorldTour teams have confirmed their use of CO inhalation for this purpose, Visma Lease-a-Bike, Israel Premier Tech and UAE Team Emirates have all used the measurement tool to legally measure the blood values and psychological benefits of altitude training in their athletes.
This use of CO inhalation for diagnostic purposes to measure blood volume will remain legal.
The Union Cycliste Internationale (UCI) banned the substance from its races in February 2025, to "protect the health of the riders." Even within a controlled medical setting, the organisation outlined that each inhalation for diagnostic purposes must be recorded in an athlete's medical record.
Despite teams including UAE insisting that "carbon monoxide [...] is a well established, safe, professional method that is backed by a very large amount of research,” the UCI maintained its position.
The practice of CO doping was first exposed by Escape Collective in July of last year, who warned that its use could infringe WADA’s rules around the artificial manipulation of blood.
The article concluded with a clear warning: “There is no hard evidence that WorldTeams are using CO inhalation for performance enhancement. But their exploration of the technique alone makes it more likely that someone else will cross that line.”
Teams were quick to respond. Uno-X Mobility told the outlet that the use of CO rebreathers for diagnostic purposes “is not something we are even considering”, and UAE Team Emirates dropped its use of the tool post-publication.
"I understand if it's misused, but I never knew that it could be misused," Jonas Vingegaard said in a press conference for Visma Lease-a-Bike last November, the rider later continuing: "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.”
The ruling on the ban on performance enhancing CO inhalation will come into effect in 2026.
