Carbon Monoxide Rebreathing: Diagnostic Tool or Doping Method?

The same gas can measure your blood or manipulate it, and the difference comes down to dose and repetition. A single, controlled breath of carbon monoxide (CO), rebreathed for a few minutes, lets a laboratory quantify how much hemoglobin a person carries, while repeated low doses instead nudge the kidney toward releasing more erythropoietin and can raise red cell mass over weeks much the way altitude does. The World Anti-Doping Agency drew its line exactly along that seam. On the 2026 Prohibited List, in force since 1 January 2026, WADA added the non-diagnostic use of carbon monoxide as a new prohibited method, M1.4, while explicitly keeping diagnostic rebreathing permitted.

One molecule, two very different jobs

Carbon monoxide binds to hemoglobin with an affinity roughly two hundred times greater than oxygen. That single fact is what makes it both a precise tracer and a hazard. When CO occupies a hemoglobin binding site, it forms carboxyhemoglobin (HbCO) and, in large amounts, starves tissues of oxygen. In small, measured amounts, that same tight binding turns CO into an almost ideal label for counting hemoglobin molecules.

The physiology only becomes a doping question when the exposure is repeated. The body reads a modest, sustained rise in carboxyhemoglobin as a drop in oxygen-carrying capacity, a signal chemically distinct from thin mountain air but interpreted by the kidney in a similar direction: make more erythropoietin, and eventually more red cells. Whether CO is a measurement or a manipulation depends entirely on how it is delivered.

How diagnostic rebreathing works

The optimized CO-rebreathing method is a dilution measurement. A subject rebreathes a small, known bolus of CO mixed with oxygen from a closed circuit, typically for several minutes, while the gas distributes through the whole circulation. As reviewed in the Journal of Applied Physiology CORP paper on total hemoglobin mass, the carboxyhemoglobin equilibrates across the blood within roughly six to eight minutes in healthy people. Blood samples taken before and after let a technician calculate how many hemoglobin molecules were available to absorb the known number of CO molecules. From that single dilution, total hemoglobin mass follows, and blood and plasma volumes can be estimated alongside it.

The clinical value is real. Total hemoglobin mass is a more stable descriptor of oxygen-carrying capacity than a concentration reading like hematocrit, which shifts with hydration. The same rebreathing chemistry, run differently, also underlies pulmonary diffusion-capacity testing, where CO uptake reports on how well gas crosses from the lung into blood. Both are recognized measurements, done once, under supervision, with a defined dose. WADA's own language reflects this: the use of carbon monoxide for controlled, diagnostic purposes, such as measurement of total hemoglobin mass by CO rebreathing or determination of pulmonary diffusion capacity, is not prohibited.

How the doping version differs

The performance version keeps the delivery apparatus but changes the intent and the schedule. Instead of one measurement, an athlete would inhale small CO doses repeatedly over days or weeks. Peer-reviewed work has explored this directly. A 2024 review in Frontiers in Physiology summarized the scientific evidence for low-dose CO inhalation as a way to raise total hemoglobin mass and endurance performance, and it laid out the proposed mechanism plainly: repeated mild elevations in carboxyhemoglobin act as a hypoxia-like stimulus that can drive erythropoiesis. That review reported that chronic intermittent low-dose CO inhalation in moderately trained athletes was associated with an increase in total hemoglobin mass of a few percent, which the authors described as comparable to the effect of altitude training.

That is the physiological equivalence WADA acted on. The performance target of intermittent CO breathing is the same red-cell expansion that altitude camps, hypoxic tents, and blood transfusion pursue by other routes, all of which fall under the broader M1 category of manipulating blood and blood components.

Why WADA classified it under M1.4

WADA evaluates a substance or method against three criteria and can prohibit it when it meets two of the three: potential to enhance performance, actual or potential health risk, and violation of the spirit of sport. Non-diagnostic CO use engages at least the first two clearly. On performance, the erythropoietic pathway is a known lever on endurance capacity. On health, WADA is direct that carbon monoxide can increase erythropoiesis under certain conditions and carries potentially fatal effects when inhaled in high or unregulated doses. There is no bright pharmacological line between a dose that trains the marrow and a dose that causes harm, and self-administered repeated exposure removes the supervision that makes the diagnostic version safe.

Placing the prohibition at M1.4, rather than banning the gas outright, is what lets the rule sit alongside legitimate testing. It targets the pattern of use, repeated non-diagnostic administration aimed at changing blood, rather than the molecule. This is why a physiology laboratory can still run a rebreathing test on Monday while the same equipment used for daily inhalation would constitute a prohibited method.

Reading the line correctly

The distinction is not diagnostic versus doping as two labels on a bottle. It is a single controlled measurement versus a repeated physiological intervention. Dose, frequency, supervision, and purpose separate the two, and the 2026 List encodes all four. For athletes and support staff, the practical reading is that legitimate hemoglobin-mass or lung-function testing remains available through qualified professionals, while acquiring or using rebreathing equipment to inhale CO outside that setting now falls squarely inside M1.4.

This article is educational and is not medical advice.