Acetazolamide is an old diuretic that has long been the standard for preventing acute mountain sickness. It is best known as Diamox; in Russian pharmacies it is sold as Diakarb, and that name alone gets 65–106 thousand searches a month on Yandex (Russia's main search engine). Most of those, of course, have nothing to do with mountains. But for runners heading to high-altitude races, to Elbrus or to the Himalayas, the question is a practical one: will the pill help, or will it get in the way of running?

On 15 October 2026 the International Journal of Sports Medicine published a double-blind placebo-controlled study that tested specifically the preventive dose rather than the treatment dose.

How it was tested

Khazaei and Tadibi (Razi University, Kermanshah, Iran) recruited 12 men — physical education students. Each completed both conditions in random order, a week apart:

acetazolamide — three doses of 125 mg over the 24 hours before the test, or placebo;
30 minutes sitting in hypoxia — 13% oxygen in the inspired air, equivalent to roughly 3800 m;
then cycling at a constant 60% of peak power — in the same hypoxia, to exhaustion.

What they found

Time to exhaustion with acetazolamide was 249.6 ± 36.2 s versus 364.7 ± 71.8 s on placebo (p < 0.001). That is 32% less.

Perceived effort was higher: on average 14.5 versus 11.8 on the Borg scale (p = 0.002) — with the pill, the work felt harder right from the start.

Yet oxygen saturation was higher — at rest in hypoxia, during exercise and at the point of exhaustion (all p ≤ 0.001). Heart rate with acetazolamide was higher only during exercise (p = 0.049).

The authors' conclusion: in acute hypoxia, a preventive dose reduces endurance and brings on a higher sense of effort sooner — despite better blood oxygen saturation.

Why more oxygen does not mean more endurance

Acetazolamide blocks carbonic anhydrase. The kidneys start excreting bicarbonate, the blood becomes slightly more acidic, and the respiratory centre responds by stepping up ventilation. That is where the higher saturation comes from — and it is the very reason the drug is given in the mountains: in effect, it speeds up acclimatisation.

But the same shift means that the blood's buffering capacity is reduced. For a runner this is the mirror image of baking soda: sodium bicarbonate before a race raises buffering capacity and helps with hard efforts (more in our look at baking soda), whereas acetazolamide lowers it.

Similar results have been seen before. In the study by Garske and colleagues (Journal of Applied Physiology, 2003), 9 healthy men given a higher dose — 500 mg twice a day, five doses in total — reached a lower maximal power output in hypoxia with the same 13% oxygen: 203 W versus 225 W on placebo. Ventilation was higher, and so was leg fatigue, but breathlessness was not.

The picture is not clear-cut, though. Fulco and colleagues at the US Army institute USARIEM (Clinical Science, 2006) gave 6 participants 250 mg three times a day and tested single-leg knee extension to exhaustion. At sea level endurance fell from 48 to 36 minutes, while at a simulated 4300 m there was no difference (17 minutes on placebo versus 20 on acetazolamide, not significant). The authors explained this by the higher saturation at altitude offsetting the harm from the acidosis.

What not to believe too firmly here

12 people, men only, not endurance athletes. Physical education students are not trail runners with years of volume behind them.

The test lasted 4–6 minutes. A day in the mountains means hours at low intensity. The 32% from a short test cannot be carried over to a 10-hour climb.

This is half an hour of acute hypoxia, not living at altitude. The drug is given to help people get through the first days of an ascent. The study does not answer how it affects performance after 3–5 days of acclimatisation.

Results differ between studies. Cycling and single-leg exercise gave different answers — which means a lot depends on how much muscle is working and how strongly ventilation limits the effort.

How to apply this

Don't take acetazolamide “for endurance”. It does not improve it — at best it doesn't hurt, at worst it reduces it. Its job is preventing altitude sickness.
Decide on it with a doctor. It is a medicine with contraindications and side effects (tingling in the fingers, frequent urination, altered taste of carbonated drinks). The 2024 Wilderness Medical Society guidelines recommend it for people at moderate and high risk of altitude sickness; the preventive adult dose they give is 125 mg every 12 hours.
The best prevention is gradual ascent. In the same guidelines, slow ascent comes first and the pill is an add-on. We covered how altitude affects training in the article on “live high, train low”.
Not for athletes subject to doping control. Acetazolamide is on the WADA list as a diuretic and masking agent (class S5) and is prohibited at all times — both in and out of competition. Without an approved therapeutic use exemption it is a violation, even at an amateur race with testing.
If you have taken it, lower your expectations for your first hard sessions at altitude. Run them by feel and heart rate rather than by your sea-level pace: at altitude your thresholds shift, and a VO₂max calculated down low does not apply up high.

The bottom line

A 2026 double-blind study: 12 men, acetazolamide 3 × 125 mg over one day, 30 minutes in hypoxia (about 3800 m), then cycling at 60% of peak power to exhaustion.
Time to exhaustion fell from 365 to 250 seconds (−32%), and perceived effort rose from 11.8 to 14.5 — despite higher saturation.
Mechanism: the drug acidifies the blood, increases breathing and raises saturation, but lowers buffering capacity.
Older studies disagree: in 2003 maximal power in hypoxia fell from 225 to 203 W; in 2006 single-leg endurance at a simulated 4300 m did not change.

Sources: Khazaei M., Tadibi V. “Acetazolamide Reduces Endurance Despite Higher Oxygen Saturation in Normobaric Hypoxia”. International Journal of Sports Medicine, 2026. DOI: 10.1055/a-2972-5318 · Garske L.A., Brown M.G., Morrison S.C. “Acetazolamide reduces exercise capacity and increases leg fatigue under hypoxic conditions”. Journal of Applied Physiology, 2003;94(3):991-996. DOI: 10.1152/japplphysiol.00746.2001 · Fulco C.S., Muza S.R., Ditzler D. et al. “Effect of acetazolamide on leg endurance exercise at sea level and simulated altitude”. Clinical Science, 2006;110(6):683-692. DOI: 10.1042/CS20050233 · Luks A.M., Beidleman B.A., Freer L. et al. “Wilderness Medical Society Clinical Practice Guidelines for the Prevention, Diagnosis, and Treatment of Acute Altitude Illness: 2024 Update”. Wilderness & Environmental Medicine, 2024;35(1_suppl):2S-19S. DOI: 10.1016/j.wem.2023.05.013