Acetazolamide (Diamox) for Altitude Sickness: Dose, Timing and Side Effects
How acetazolamide helps prevent altitude sickness, how to take it, the side effects to expect and who it isn't suitable for.
Part of the Access Doctor Altitude Sickness Guide.
Key fact: The usual dose to help prevent altitude sickness is 125mg twice a day (half a 250mg tablet), started 1 to 2 days before you go up and continued for at least 2 days after reaching your highest point.
Acetazolamide, also known by the brand name Diamox, is the medicine UK travel health services recommend when tablets are needed to help prevent altitude sickness. It speeds up acclimatisation; it doesn’t mask symptoms.
It isn’t licensed for altitude sickness in the UK, so it’s prescribed “off-label”. This is common and accepted practice, supported by NaTHNaC and the NHS Specialist Pharmacy Service. For the condition itself, see our guide to altitude sickness.
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Start Altitude Sickness Consultation →How Acetazolamide Works
Acetazolamide makes your kidneys pass more bicarbonate into your urine, which makes your blood slightly more acidic. Your body responds by breathing faster and deeper, so you take in more oxygen, particularly at night. That’s the same change your body makes naturally over several days at altitude, so acetazolamide helps you acclimatise sooner.
It’s an aid, not a substitute. Acetazolamide lowers your risk but doesn’t remove it. You still need to go up gradually, and you can still get altitude sickness while taking it.
Dose and Timing
| What to do | |
|---|---|
| Dose | 125mg twice a day: half a 250mg tablet in the morning and half in the early evening |
| Start | 1 to 2 days before you start going up |
| Stop | At least 2 days after reaching your highest point, or when you start coming down from a summit if you have no symptoms |
| Evening dose | Take it at dinnertime, not bedtime, because it makes you pass more urine |
| Tablets needed | One 250mg tablet a day, plus 2 for the trial |
Your prescriber may advise a different dose for your trip. Take the dose on your prescription label.
The 250mg twice a day dose
You may read about a higher dose: a whole 250mg tablet twice a day. Doctors use it to treat altitude sickness that has already started, alongside rest and stopping the ascent. Some expedition doctors also use it for prevention on very fast ascents.
We don’t supply acetazolamide at this dose. Access Doctor prescribes it for prevention only, at half a tablet twice a day. Don’t increase your dose yourself to treat symptoms or to keep climbing. If you’re planning a very fast ascent, see a travel clinic or expedition doctor before you go.
The Trial Dose
Take acetazolamide for 1 to 2 days at home, a couple of weeks before you travel. Its side effects can feel like mild altitude sickness, so it helps to know how it affects you before you’re on the mountain. A trial also shows up any allergic reaction while you’re still near medical help.
Side Effects
Side effects are common but usually mild, and they stop when you stop the tablets:
- Tingling in the fingers, toes and face
- Passing more urine, especially in the first few days
- Taste changes: fizzy drinks can taste flat or metallic
- Feeling sick, headache, dizziness or tiredness
Stop taking it and get medical help if you get a rash, blistering or peeling skin, a sore throat with fever, unusual bruising or bleeding, yellow skin or eyes, sudden changes in your eyesight, or swelling of the face or throat.
Acetazolamide can make your skin more sensitive to the sun, which is already strong at altitude, so use high-factor sunscreen. Don’t drive if it makes you drowsy or dizzy. Report suspected side effects through the MHRA Yellow Card scheme.
Who Shouldn’t Take It
- Anyone who has had a severe allergic reaction to a sulfonamide (“sulfa”) medicine, such as the antibiotic co-trimoxazole
- Women who are pregnant, especially in the first 3 months; it’s used with great caution when breastfeeding
- People with significant kidney or liver disease, including cirrhosis
- People with low sodium or potassium levels, or adrenal gland failure (Addison’s disease)
Tell your prescriber if you’ve had kidney stones, have diabetes, gout or a long-term lung condition, or are over 65.
Other Medicines
- Aspirin in regular or high doses: the combination can cause serious side effects
- Lithium, phenytoin, carbamazepine and ciclosporin: their levels can change
- Topiramate and zonisamide: these work in a similar way and shouldn’t be combined with it
- Diabetes medicines, blood thinners, digoxin and blood pressure tablets: doses may need checking
On the Mountain
- Keep drinking: you’ll pass more urine, and the air at altitude is dry
- Carry your tablets in your day bag, in their labelled box
- Symptoms despite tablets? Treat them as altitude sickness: stop, rest, and go down if you’re not improving
For the rules of safe ascent, see how to prevent altitude sickness.
Going Above 2,500m?
Tell us about your trip and your health. A UK pharmacist prescriber checks whether preventive treatment is suitable for you. Approved before 3pm, Mon–Fri? Dispatched the same day.
Start Altitude Sickness Consultation →Frequently Asked Questions
What is the dose of acetazolamide for altitude sickness?
The usual dose to help prevent altitude sickness is 125mg twice a day, which is half a 250mg tablet in the morning and half in the early evening. Take the dose your prescriber gives you.
When should I start taking acetazolamide for altitude?
Start 1 to 2 days before you begin going up, and continue for at least 2 days after reaching your highest point. If you come down from a summit without symptoms, you can stop when you start descending.
Is Diamox the same as acetazolamide?
Yes. Diamox is a brand name for acetazolamide. The tablets we supply are generic acetazolamide 250mg, which contain the same medicine.
Is acetazolamide licensed for altitude sickness in the UK?
No. No medicine is licensed in the UK to prevent altitude sickness, so acetazolamide is prescribed off-label. UK travel health services, including NaTHNaC, recommend it when a preventive medicine is needed.
Why does acetazolamide cause tingling?
Tingling in the fingers, toes and face is a common, harmless side effect caused by the change in your blood's acid balance. It goes away when you stop the tablets.
Can I drink alcohol with acetazolamide?
It's best to avoid alcohol at altitude. It dehydrates you, disturbs sleep and can hide the early symptoms of altitude sickness.
Can I take acetazolamide if I'm allergic to sulfa antibiotics?
Not if you've had a severe reaction, such as anaphylaxis or a blistering rash. Acetazolamide is related to sulfonamide medicines. With a milder past reaction, a prescriber may consider a supervised trial before travel.
Does acetazolamide treat altitude sickness once it starts?
Doctors sometimes use a higher dose, 250mg twice a day, to treat altitude sickness, alongside rest and stopping the ascent. We don't supply it for treatment, only for prevention at half a tablet twice a day. Severe altitude sickness needs immediate descent.
References
- NHS Specialist Pharmacy Service. Using medicines for preventing acute mountain sickness. sps.nhs.uk
- NaTHNaC, TravelHealthPro. Altitude illness. travelhealthpro.org.uk
- Electronic Medicines Compendium. Acetazolamide 250mg tablets: Summary of Product Characteristics. medicines.org.uk
- Luks AM, et al. Wilderness & Environmental Medicine. Wilderness Medical Society clinical practice guidelines for the prevention, diagnosis and treatment of acute altitude illness: 2024 update. journals.sagepub.com
- MHRA. Yellow Card scheme. yellowcard.mhra.gov.uk
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Prescription medicines require a consultation before they can be dispensed. Always consult a qualified healthcare professional for diagnosis and treatment. In a medical emergency, call 999.