Carbon monoxide is a banned method from 1 January. The test that made it routine is not

The World Anti-Doping Agency's 2026 Prohibited List came into force on 1 January, and its headline addition is a gas that has been sitting legally in elite endurance sport for years. Non-diagnostic use of carbon monoxide has been added to the prohibited methods as a new section, M1.4 [s1].

WADA's Executive Committee approved the list on 11 September 2025 [s1], and the list was published on 28 October 2025 ahead of its 1 January effective date [s2]. The rationale given is straightforward: carbon monoxide can increase erythropoiesis — red blood cell production — under certain conditions [s3].

What makes this a harder rule than it looks is the carve-out. Diagnostic applications remain permitted, specifically the measurement of haemoglobin mass and testing of pulmonary capacity [s3]. The banned thing and the allowed thing use the same gas, the same equipment, and frequently the same people.

What the legitimate test actually is

The permitted procedure is a haemoglobin mass measurement, used to assess how an athlete has responded to altitude training. An athlete inhales a single bolus dose of roughly 75 to 125 millilitres of carbon monoxide — around 1.25 millilitres per kilogram of body weight — and rebreathes through a closed-circuit spirometer for two minutes to let the gas mix [s4].

That raises the carbon monoxide saturation of the blood to about 5 per cent [s4]. For scale, a normal blood level is around 0.5 per cent and a smoker averages about 3 per cent [s4]. The test is known colloquially as the "bong test," and it is not new: Laura Lewis, USADA's director of science, has described running it at every stage of the 2010 Tour Down Under and at altitude locations including the Stelvio Pass [s4].

What the prohibited use is thought to be

The performance question comes from a different protocol entirely. A 2020 study by Professor Walter Schmidt had subjects inhale small amounts of carbon monoxide five times a day for three weeks. That reduced oxygen transport by approximately 5 per cent — an effect the researchers compared to training at 2,500 metres of altitude — and produced roughly a 5 per cent increase in total haemoglobin [s4].

This is worth stating carefully. That is one experimental study, in a small group, testing a deliberate repeated-exposure protocol. It establishes that repeated low-dose carbon monoxide can shift a haematological marker. It does not establish a race result, and it is not evidence about what any particular team or athlete has done.

The practice became a public argument during the 2024 Tour de France [s4]. As of September 2024 the method was not prohibited by WADA, though the agency had indicated it would look further into a possible ban [s4].

The safety argument sits underneath the fairness one

Carbon monoxide binds haemoglobin with a much higher affinity than oxygen does [s4]. That is precisely why the diagnostic test works, and it is also why repeated dosing outside a controlled setting is the part anti-doping scientists talk about first. Lewis has warned that the risks become dangerous if the technique spreads without the proper machines and expertise [s4].

The 2026 list codifies that distinction rather than resolving it. A single controlled bolus for measurement is permitted. Repeated non-diagnostic inhalation is not.

The rest of the 2026 list

Carbon monoxide is the structural change, but several other modifications took effect on the same date.

Salmeterol, an inhaled asthma medication, has had its dosing intervals changed to avoid potential ergogenic effects, while the maximum permitted daily dose is unchanged [s1]. In practice that means no more than 100 micrograms inhaled in any 8-hour period, with the 24-hour total still 200 micrograms [s3].

Withdrawal of blood or blood components is now explicitly prohibited except for two purposes: analytical purposes such as medical tests or doping control, and donation performed at a collection centre accredited by the relevant regulatory authority [s3].

Under gene and cell doping, cell components — nuclei and organelles such as mitochondria and ribosomes — have been added to the existing prohibition on using normal or genetically modified cells [s1].

The glucocorticoids washout table has been clarified to note that sustained-release formulations may produce detectable levels past the washout period because of prolonged systemic absorption [s3]. That is a practical warning for athletes who receive a legitimate injection and assume a standard clearance window applies.

Further examples and clarifications have also been added across four substance classes — S1 anabolic agents, S2 peptide hormones and growth factors, S4 hormone and metabolic modulators, and S6 stimulants [s1].

What to watch

The list applies to all signatories from 1 January [s1], which places the carbon monoxide rule in force weeks before the Winter Olympic programme begins. The open question is not the rule's wording but its administration: a permitted diagnostic protocol and a prohibited training protocol differ in frequency and intent rather than in substance, and frequency and intent are harder to test for than a metabolite.

Sources

1. [s1] WADA's 2026 Prohibited List comes into force on 1 January. International Paralympic Committee, 30 December 2025. https://www.paralympic.org/news/wada-2026-prohibited-list-january
2. [s2] World Anti-Doping Agency Prohibited List for 2026 published. International Tennis Integrity Agency, 28 October 2025. https://www.itia.tennis/news/itia-news/world-anti-doping-agency-prohibited-list-for-2026-published/
3. [s3] Athlete Advisory: What's New on the 2026 WADA Prohibited List? U.S. Anti-Doping Agency, January 2026. https://www.usada.org/spirit-of-sport/2026-wada-prohibited-list/
4. [s4] Analysis: The history of carbon monoxide rebreathing and whether it should be banned. BikeRadar, September 2024. https://www.bikeradar.com/features/opinion/usada-carbon-monoxide-rebreathing