Carbon Monoxide Rebreathing: Diagnostic Tool or Doping Method?

A single supervised breath of carbon monoxide, held in a closed loop for a few minutes, is a laboratory measurement; the same gas breathed in small amounts day after day is a way to build blood, and on its 2026 Prohibited List the World Anti-Doping Agency (WADA) drew the line precisely between those two uses. Effective 1 January 2026, WADA added the non-diagnostic use of carbon monoxide as a new prohibited method, M1.4, while stating plainly that using carbon monoxide to measure total hemoglobin mass or lung diffusion capacity remains allowed. The rule bans a pattern of use, not a molecule.

Key points#

• One controlled dose of carbon monoxide (CO) measures how much hemoglobin a person carries; many small doses over weeks can raise red cell mass, similar to altitude.
• WADA's 2026 List prohibits the second use as method M1.4 but keeps supervised diagnostic rebreathing permitted.
• Dose, frequency, supervision, and purpose are what separate a lab test from a prohibited method.
• CO binds hemoglobin about 200 times more tightly than oxygen, which is what makes it both an exact tracer and a real hazard.

Why one gas landed on the 2026 List#

For years, carbon monoxide sat comfortably inside sports science as a measuring instrument. What changed is that researchers began publishing on the opposite use: breathing tiny amounts of CO repeatedly to coax the body into making more red blood cells. Once that method moved from theory toward practice, anti-doping regulators had to decide whether an existing measurement tool could also become a performance aid. Their answer, on the 2026 List, was to name the abusive pattern rather than outlaw the gas, so that legitimate physiology labs could keep working while athletes could not repurpose the same equipment as a training device.

The chemistry behind a ruler and a risk#

Carbon monoxide binds to hemoglobin roughly 200 times more avidly than oxygen does. That one property explains everything that follows. When CO occupies a binding site it forms carboxyhemoglobin (HbCO); in large amounts it crowds out oxygen and can be lethal, which is the familiar danger of a faulty furnace. In tiny, measured amounts, that same tight grip makes CO an almost perfect label for counting hemoglobin, because nearly every molecule it meets holds on.

The trouble begins only when the dosing is repeated. A modest, sustained rise in carboxyhemoglobin reads to the body like thinner air. The kidney responds the way it would at altitude: it releases more erythropoietin, and over weeks the marrow builds more red cells. The chemistry is different from a mountain camp, but the downstream signal points the same way. So whether CO is a ruler or a lever depends entirely on how much you take, how often, and under whose supervision.

Measurement: one breath, counted#

Diagnostic CO rebreathing is a dilution test. You rebreathe a small, known bolus of CO mixed with oxygen from a sealed circuit, usually for several minutes, while the gas spreads evenly through the circulation. As a Journal of Applied Physiology methods review on total hemoglobin mass describes, the carboxyhemoglobin typically equilibrates across the blood within about six to eight minutes in healthy people. Blood drawn before and after shows how much the known dose of CO was diluted, and from that a technician back-calculates the total mass of hemoglobin available to absorb it. Blood and plasma volumes can be estimated from the same run.

The clinical payoff is genuine. Total hemoglobin mass is a steadier picture of oxygen-carrying capacity than a concentration such as hematocrit, which rises and falls with hydration. The same CO chemistry, arranged differently, drives pulmonary diffusion-capacity testing, where CO uptake reports how readily gas crosses from lung into blood. Both are established, one-time, supervised measurements with a defined dose, and WADA's own text carves them out: controlled diagnostic use, including hemoglobin-mass rebreathing and lung diffusion testing, is not prohibited.

Manipulation: many breaths, over weeks#

The performance version keeps the apparatus and changes the schedule and the goal. Rather than a single measurement, an athlete would inhale small CO doses again and again across days or weeks. A 2024 review in Frontiers in Physiology gathered the evidence for this approach and set out the proposed mechanism directly: repeated mild rises in carboxyhemoglobin act as a hypoxia-like stimulus that can drive red cell production. The review reported that chronic intermittent low-dose CO inhalation in moderately trained athletes was associated with a rise in total hemoglobin mass of a few percent, an effect the authors compared to altitude training.

That is the equivalence WADA acted on. The target of intermittent CO breathing is the same red-cell expansion that hypoxic tents, altitude camps, and blood transfusion each pursue by their own route, all of which already sit under the broader M1 heading for manipulation of blood and blood components.

The three-part test WADA applied#

WADA can prohibit a substance or method when it satisfies two of three criteria: potential to enhance performance, actual or potential harm to health, and violation of the spirit of sport. Non-diagnostic CO use clearly clears the first two. On performance, boosting red cell mass is a well-mapped lever on endurance. On safety, WADA states that carbon monoxide can increase erythropoiesis under certain conditions and carries potentially fatal effects at high or unregulated doses. There is no clean pharmacological gap between a dose that stimulates the marrow and a dose that injures, and unsupervised self-dosing strips away the very oversight that keeps the diagnostic test safe.

Placing the ban at M1.4, rather than outlawing the gas, is what lets the rule coexist with legitimate testing. It targets the behavior (repeated non-diagnostic administration meant to change the blood) instead of the chemical. That is why a lab can run a rebreathing test on a Monday while the identical equipment, used for daily inhalation, would be a prohibited method.

What this means in practice#

The real distinction is not two labels on a canister. It is one controlled measurement versus a repeated physiological intervention, and four variables draw the boundary: dose, frequency, supervision, and purpose. The 2026 List encodes all four. If you are an athlete or support staff, the practical reading is straightforward. Hemoglobin-mass and lung-function testing stay available to you through qualified professionals, while acquiring or using rebreathing gear to inhale CO outside that clinical setting now falls squarely within M1.4.