Medical disclaimer. Acetazolamide (Diamox) requires a prescription. Information in this article is for educational purposes only. Consult your doctor before using this medication, especially if you have other medical conditions or are taking other treatments.
Diamox is the most commonly prescribed medication for altitude sickness prevention — and also one of the most misunderstood. It is not a "magic pill" that lets you ascend faster than your body can adapt. But when used correctly — on your doctor's advice, at the right dosage — it significantly reduces the risk of AMS on peaks that reach and exceed 3,000 m such as Kilimanjaro (5,895 m) or Mount Elbrus (5,642 m).
You can check the oxygen level and physiological risk at your target altitude in advance with the Oxymeter calculator.
How Acetazolamide Works at Altitude
Acetazolamide is a carbonic anhydrase inhibitor: it blocks the enzyme that, in the kidneys, exchanges bicarbonate (HCO₃⁻) and hydrogen ions. The result is increased bicarbonate excretion in the urine — a mild metabolic acidosis.
Why does this help at altitude? Under normal conditions, hypoxia-driven hyperventilation lowers CO₂ in the blood (respiratory alkalosis) and paradoxically inhibits the reflex to hyperventilate further — a biological brake that limits the effectiveness of the adaptive response. The metabolic acidosis induced by Diamox "compensates" for this alkalosis, allowing the body to continue hyperventilating effectively and thereby extract more oxygen from the thin air.
In practice: with acetazolamide you breathe more deeply and more frequently even at night, SpO₂ improves by 2–5 percentage points compared with no medication at the same altitude, and the risk of AMS is reduced.
Recommended Dosage
| Use | Dosage | When to Start | Duration |
|---|---|---|---|
| Prophylaxis (prevention) | 125–250 mg × 2/day | 24 hours before ascent | Until 48 hours after reaching maximum altitude |
| Treatment (established AMS) | 250 mg × 2/day | At symptom onset | Until resolution + 24 hours |
| HACE/HAPE prophylaxis (high-risk individuals) | 250 mg × 2/day | 24–48 hours before | Throughout the stay at altitude |
Data: WMS Clinical Practice Guidelines 2024. Dosage must be agreed with your physician.
125 mg or 250 mg?
WMS 2024 guidelines indicate that 125 mg twice daily provides effective prophylaxis with fewer side effects than 250 mg. For most healthy climbers on peaks up to 5,000–5,500 m, 125 mg is the recommended starting point. Higher doses (250 mg) are reserved for individuals at high personal risk or very rapid ascents.
Tolerance test
If you have never used acetazolamide, do a low-altitude tolerance test 1–2 weeks before your trip: take 125 mg in the evening and assess your response the following day. The side effects — paresthesia, increased urination — are unpleasant but benign. If you tolerate it well, you can use the medication with greater confidence at altitude.
Side Effects and What to Expect
Increased urination
Acetazolamide is a diuretic: it increases urine output. This is not a side effect to fight against — it is part of the mechanism of action. At altitude, where dehydration is already a risk (dry air, hyperventilation), this makes adequate hydration even more important: at least 3–4 litres of water per day above 3,000 m.
Paresthesia (tingling)
Tingling in the fingers, toes, lips, and tongue is the most common side effect — it affects 40–70% of users. It is completely benign, caused by carbonic anhydrase inhibition in peripheral nerves, and resolves when the medication is stopped. Many climbers treat it as confirmation that the drug is working.
Altered taste of carbonated drinks
Acetazolamide makes carbonated drinks (cola, sparkling water, beer) taste flat and unpleasant. At altitude this is rarely a meaningful problem — alcohol is best avoided in any case.
Photosensitivity
Rare but possible. Use high sun protection (SPF 50+) at altitude — recommended regardless given the increased UV radiation at elevation.
Contraindications and Interactions
| Situation | Detail |
|---|---|
| Sulfonamide allergy | Absolute contraindication — cross-reactivity with acetazolamide |
| Severe renal insufficiency | Contraindicated — drug accumulation |
| Hyponatraemia / hypokalaemia | Risk of worsening |
| Severe hepatic cirrhosis | Risk of hepatic encephalopathy |
| First trimester of pregnancy | Contraindicated (teratogenicity reported in animal studies) |
| Breastfeeding | Not recommended |
| Lithium | Interaction — reduces lithium excretion |
| Oral anticoagulants | Monitor prothrombin time |
| Methotrexate | Reduced renal excretion |
This list is not exhaustive. Always show the medication's full prescribing information to your doctor before any expedition.
When to Start, When to Stop
Start: 24 hours before ascending to the critical altitude (generally above 2,500–3,000 m). If you are flying or travelling rapidly directly to a base camp above 4,000 m, begin 48 hours before.
Continue: throughout your stay at altitude, until 48 hours after reaching maximum altitude or after descent.
Stop: do not discontinue abruptly at altitude without descending — sudden cessation can cause a rebound respiratory alkalosis and temporarily worsen symptoms.
Do not use: if you already have clear moderate-to-severe AMS symptoms. In that case, the priority is descent and specific treatment. Diamox is not a substitute for descent in HACE/HAPE.
Diamox Is Not a Substitute for Acclimatization
The most important point in this entire article: Diamox reduces the risk of AMS, but does not eliminate the need for gradual ascent. A climber taking acetazolamide who ascends too quickly can still develop altitude sickness — and in the worst case, HACE or HAPE.
Use it as an additional tool in your safety strategy, not as a shortcut. On peaks such as Aconcagua or Kilimanjaro, respecting acclimatization timelines remains the single most effective protection.
Frequently Asked Questions
Does Diamox really prevent altitude sickness?
Yes. Clinical trials show a 30–60% reduction in AMS incidence compared to placebo. It is not, however, a substitute for gradual ascent: even with prophylaxis, following acclimatization rates is indispensable.
What is the correct Diamox dosage?
Prophylaxis: 125–250 mg twice daily, starting 24 hours before ascent and continuing for 48 hours after reaching maximum altitude. AMS treatment: 250 mg twice daily. Prescription is required.
What are the side effects?
The most common are increased urination and tingling in the hands, feet, and lips (paresthesia). Both are benign. Carbonated drinks may taste flat. Rarely: nausea, photosensitivity. Contraindicated in sulfonamide allergy.
Who cannot take Diamox?
Those with sulfonamide allergy, severe renal insufficiency, severe hepatic cirrhosis, significant hyponatraemia or hypokalaemia, or who are in the first trimester of pregnancy. Consult your doctor regarding any other medical condition or ongoing medication.
Before planning your ascent, check the oxygen levels and physiological risk with the Oxymeter calculator.
Learn more about managing altitude emergencies in the article on HACE and HAPE — the severe forms of altitude illness that Diamox helps prevent.
