Diamox (Acetazolamide) on Nepal Treks

Acetazolamide, sold under the brand name Diamox, is the most commonly discussed drug for altitude illness, and the most commonly misunderstood. It can help prevent and treat acute mountain sickness, but it does not let you ignore the ascent rate, and it does nothing to make a trekker with severe illness safe to stay high. It is a prescription drug in many countries, so the decision to take it belongs with a doctor who knows your health. This guide explains what the CDC and the Wilderness Medical Society say about it, how that fits the Everest Base Camp and Annapurna Circuit plans and what questions to bring to your appointment.

This is general information, not medical advice. The doses below are those the CDC Yellow Book lists for adults and are included so you can talk about them with a clinician, not so you can self-prescribe.

What it is for

Acetazolamide is used for two purposes in altitude medicine: preventing acute mountain sickness (AMS) when ascent is fast or when someone has been ill at altitude before, and treating mild to moderate AMS. It speeds acclimatisation. It is not a treatment for the severe forms. For high-altitude pulmonary edema (HAPE) and cerebral edema (HACE) the answer is descent, oxygen and other drugs, as in our guide to HAPE and HACE warning signs.

The CDC Yellow Book and the Wilderness Medical Society guidance it cites make slow ascent the first line of prevention. Above 3,000 m, that means gaining no more than 500 m of sleeping altitude per night and adding an extra night for every 1,000 m gained. The NHS gives similar advice and adds a rest day every 3 to 4 days at 3,000 m or more. If you follow a schedule like the one in our acclimatisation guide, many trekkers need no medication. Acetazolamide is a back-up for people who cannot keep to the schedule or who know they are susceptible.

Doses the CDC lists

For adults, the CDC lists:

• Prevention: 125 mg every 12 hours, starting the day before ascent and continuing for the first two days at altitude, or longer if you keep ascending. For people over 100 kg, 250 mg twice daily.
• Treatment of AMS: 250 mg twice daily.
• Children: prevention 1.25 mg/kg every 12 hours, up to 125 mg.

These are guideline doses, and a doctor may adjust them, so use the figures to frame a conversation. Ask about timing and how long to continue, and ask what to do if you have to descend.

Side effects and cautions

The CDC mentions two effects you are likely to notice. One is paraesthesia, tingling in fingers and toes, at higher doses. The other is that carbonated drinks taste flat or odd, because the drug inhibits carbonic anhydrase. Tingling is annoying but not usually dangerous; tell your doctor if it is troublesome.

On allergy, the CDC says allergic reactions are uncommon and that a sulfa-antibiotic allergy does not preclude using acetazolamide, though caution applies to a history of anaphylaxis to any drug or multiple drug allergies. Do not take it on the strength of this article if you have those histories. The CDC also lists a number of conditions that are contraindications or call for caution for high-altitude travel in general, including poorly controlled asthma, severe COPD, decompensated heart failure, a recent heart attack or stroke, sickle cell anaemia and high-risk pregnancy. If any applies to you, ask your doctor about the trip itself, not just the pill.

How the pill fits a Nepal itinerary

Our two standard plans show where the risk is highest.

On the Everest Base Camp route the walk starts at Lukla and puts you at Phakding (2,610 m) on day 1 and Namche Bazaar (3,440 m) on day 2. The flight from Kathmandu (about 1,400 m) lands you at Lukla, roughly 2,860 m, in 35 to 40 minutes, so your body goes from low altitude to the edge of the zone where the 500 m rule starts within a morning. From Namche the plan climbs to Tengboche (3,860 m), Dingboche (4,410 m), Lobuche (4,940 m) and Gorak Shep (5,164 m), with rest days at Namche and Dingboche and an easier return.

On the Annapurna Circuit the plan climbs from Dharapani (1,860 m) to Chame (2,670 m), Upper Pisang (3,300 m) and Manang (3,540 m), rests, then goes to Yak Kharka (4,050 m), Thorong Phedi (4,540 m) and over the pass at 5,416 m to Muktinath (3,800 m). Thorong La is a big day and the descent on the far side is steep and long, so a person who is unwell on the climb has options on the way down, but not quickly.

In both cases the drug can reduce the odds of AMS if you take it as directed, but it does not change the physical fact that you are higher. If you develop symptoms on medication, descend or rest as you would without it.

If you take it and still feel unwell

Taking a prophylactic dose does not mean you are immune, and the biggest risk is a false sense of security. Treat any new headache, nausea, dizziness or poor sleep exactly as you would without the drug. The NHS advice is to rest at the same altitude, not to go higher until you feel better and to descend about 300 to 1,000 m if symptoms are getting worse or have not improved after a day. Mild symptoms usually improve within one to three days. Tell your partner or guide the same evening, not the next morning, so someone else is watching how you look and behave. Confusion, loss of balance, breathlessness at rest or a wet cough are not mild altitude sickness and mean going down at once, as set out in the HAPE and HACE guide above.

It also helps to agree a decision rule with your partners before the trek, when nobody is tired or proud. For example: if anyone has a headache plus another symptom that has not eased after a night, the group stays put a day; if anyone is worse after that day, the group descends together. A rule written in Kathmandu is much easier to follow at 4,400 m. Our partner guide covers how to handle the conversation when one person has to turn back.

What the drug does not do

1. It does not replace rest days. The schedule is the main protection.
2. It does not cure HAPE or HACE. Those need descent and oxygen.
3. It does not make a late start safe. Pushing on because you are "covered" is the common misuse.
4. It may not stop all symptoms. If you have a persistent or worsening headache, vomiting, confusion or breathlessness at rest, descend and seek help regardless.

Dexamethasone is a different drug used for treatment, not routine prevention. The CDC describes it as off-label for altitude illness, reserved for treatment, and rarely needed beyond a day or two. It can cause a mild rebound if stopped at altitude before you have acclimatised. Whether to carry it is a decision for your doctor, with instructions, and it is not something to hand around between trekkers.

What to ask your doctor

• Do I have a reason to take acetazolamide for prevention, given my history and the ascent plan?
• Is it safe with my other medicines and my allergies?
• What dose and timing would you suggest, and when would I stop?
• Should I try it at home first to see how I respond?
• Should I carry dexamethasone or another medicine in a kit, and when do I use it?
• What would make you advise me not to go at all?

Bring the route plan, the day-by-day altitudes and your insurer's emergency number to the appointment.

Practical points for the trail

• Carry it in the original packaging with the prescription or a letter from your doctor, and keep some in your daypack, not only in the duffel carried by a porter.
• Tell your guide and partner that you are taking it and why, so they can interpret your symptoms.
• Keep drinking water. The NHS advises staying hydrated and avoiding alcohol while ascending.
• Do not share tablets. Someone else's allergies, doses and conditions are not yours.
• Pair it with the basics. A slow plan, rest days and honest reporting of symptoms do far more.

For where help is on the trail, see the HRA clinics guide, and for the wider picture start with altitude sickness basics.