Cusco, La Paz, the Everest Base Camp trek and Kilimanjaro share one risk: lower oxygen pressure in the air, which your body needs time to adjust to. Altitude illness can affect anyone, whatever their age or fitness.

How high is high?

The Government of Canada's advice on altitude illness starts at 2,000 m. CATMAT, the Public Health Agency of Canada's travel advisory committee, notes that some people get symptoms from 2,500 m, and the risk rises the higher and faster you climb. Popular destinations include:

Cusco (3,400 m) and Machu Picchu (2,430 m), Peru
La Paz, Bolivia (3,640 m) and Quito, Ecuador (2,850 m)
Leh, Ladakh, India (about 3,500 m) and Lhasa, Tibet (about 3,700 m)
Nepal's Annapurna circuit (Thorong La pass, 5,416 m) and Everest Base Camp (about 5,400 m, says the US CDC)
Kilimanjaro, Tanzania (summit 5,895 m)

CATMAT's altitude statement dates from 2007, so this guide also cites the Wilderness Medical Society's 2024 guidelines (WMS).

Know the signs

Acute mountain sickness (AMS) is the milder, common form: headache, nausea or vomiting, tiredness, poor appetite and dizziness, starting a few hours to a few days after you arrive.

High-altitude cerebral edema (HACE), swelling of the brain, is the extreme end of AMS: loss of coordination (trouble walking heel-to-toe), confusion, drowsiness and extreme tiredness.

High-altitude pulmonary edema (HAPE), fluid in the lungs, is the most common cause of death from high altitude, says CATMAT. It usually starts 2 to 4 days after going above 2,500 m: a new or persistent cough, unusual breathlessness on effort, then breathlessness at rest and bluish lips.

The working rule: treat feeling unwell in the first few days after going higher as altitude illness until something else is clearly to blame. The WMS says people who feel ill within 3 days of going above 2,500 m likely have AMS.

How to go up safely

Canada's advice, from travel.gc.ca and CATMAT:

Avoid going quickly to a sleeping altitude above 3,000 m.
Spend 2 to 3 nights at 2,500 to 3,000 m before going higher, then an extra night for every further 600 to 900 m.
Climb high, sleep low: the height you sleep at matters more than the day's highest point.
Don't sleep any higher while symptoms are getting worse.
For the first 48 hours, avoid alcohol, sleeping pills and over-exertion.

Above 3,000 m, CATMAT's rule is to sleep no more than 300 m higher each night, with a rest day every 2 or 3 days. The WMS allows up to 500 m a day, resting every 3 to 4 days; this guide follows Canada.

Flying in versus trekking

Flying into Cusco, La Paz, Lhasa or Leh takes you above 3,000 m in less than a day, a rapid ascent that CATMAT lists as a reason to consider acetazolamide. The US CDC adds that people arriving in Cusco or La Paz can sometimes sleep a night or two lower than the airport first.

Treks go much higher. Canada's travel advice notes that many popular trails in Nepal climb above 5,500 m, that India has no commercial mountain rescue above 3,000 m, and that emergency help on Kilimanjaro is severely limited. Buy insurance that covers medical evacuation; the Government of Canada can't pay for one.

Acetazolamide (Diamox)

Acetazolamide speeds up acclimatization. It is prescription-only in Canada, and the Diamox brand is no longer sold here, so pharmacies dispense the generic. Its Canadian label lists other uses, such as glaucoma, so altitude use follows CATMAT rather than the label.

Who it is for

CATMAT lists three reasons to use it: a rapid ascent (under a day) to above 3,000 m, a big one-day jump in sleeping height (such as 4,000 m to 5,000 m), or past AMS or HAPE. The WMS says to use it alongside a gradual ascent, not instead of one.

Doses the guidelines state

CATMAT's statement gives 125 mg to 250 mg twice a day for prevention, from 24 hours before ascent through the first 2 days at high altitude. The WMS recommends 125 mg every 12 hours from the night before; on an up-and-down climb such as Kilimanjaro, it is stopped once you start down if you have no symptoms. For treatment, CATMAT gives 250 mg, then 250 mg 8 hours later; the WMS gives 250 mg every 12 hours. Children's doses are based on weight. Your physician decides whether you need it and at what dose.

Side effects

CATMAT lists tingling, needing to urinate more often, nausea, drowsiness, impotence and nearsightedness, and fizzy drinks can taste different. The 2025 Canadian monograph also warns of serious reactions: a severe skin reaction, or sudden breathing trouble from fluid in the lungs. If either happens, stop taking it and get medical help right away.

If you have a sulfa allergy

CATMAT says a sulfa allergy does not appear to predict a reaction to acetazolamide, and it should not be withheld. The WMS calls the risk extremely low and says a supervised trial dose before the trip can be considered, but rules it out after a severe allergic reaction (anaphylaxis) to a sulfa drug, or Stevens-Johnson syndrome.

Dexamethasone and nifedipine

Neither is routine. CATMAT keeps dexamethasone, a corticosteroid, for treating AMS or for prevention in people who can't take acetazolamide; stopping it without going down usually brings symptoms back. Nifedipine has not been found to help AMS. CATMAT reserves it for people prone to HAPE who are going where oxygen and descent may be limited, and it never replaces a slow ascent.

What doesn't work

Ginkgo biloba. CATMAT says it is no better than placebo, and the WMS advises against it.
Coca tea or leaves. The WMS says they haven't been properly studied and shouldn't replace proven measures.
Forcing extra fluids. Canada advises drinking more water as you climb, but the WMS says forced hydration has never been shown to prevent altitude illness.
Canned oxygen, oxygen bars and supplements. The WMS says these have no role in prevention or lack evidence.

Children, pregnancy, and heart or lung conditions

Children: studies suggest they get AMS at least as often as adults, and young children's signs (nausea, vomiting, irritability) are easily mistaken for other illness. CATMAT's statement on children says experts recommend going no higher than 2,500 m with them, and climbing 300 m a day, with a rest day every 1,000 m, if you go higher to sleep. Acetazolamide can be considered for an unavoidable ascent, but Health Canada hasn't authorized it for children, so the physician decides.

Pregnancy: the Government of Canada advises not travelling above 3,658 m while pregnant, or above 2,500 m in a high-risk or late pregnancy. The US CDC suggests not sleeping above 3,050 m; this guide follows Canada. The Canadian label says not to use acetazolamide in pregnancy, and CATMAT says to use it only if absolutely necessary.

Heart or lung problems, or anemia: Canada advises seeing a health care provider, ideally 6 weeks before you go. CATMAT notes that altitude can worsen heart and lung disease, that asthma should be well controlled first, and that significant heart or lung disease may need assessing, including for oxygen.

If symptoms start: stop, then go down

Stop climbing and rest. Pain relievers can ease a mild headache.
If symptoms get worse while you rest, go down. Descending at least 500 m below where they began usually reverses AMS.
Loss of coordination, confusion or breathlessness at rest is an emergency: start down right away with minimal exertion, call for help, and don't let the sick person go alone.
Go back up only when you feel better and have no symptoms.

Oxygen and portable pressure bags can buy time, but CATMAT calls descent the treatment of choice.

See our Peru and Nepal trekking guides, and vaccine advice for Peru, Nepal, Tanzania and India. A $110 virtual consultation with a physician licensed in Ontario can cover your route and whether acetazolamide suits you. You need to be in Ontario for the video call, and prescriptions go to our Toronto pharmacy, Cloud Pharmacy at 55 Dundas St E. Book a consultation.

Frequently asked questions

Can I buy Diamox without a prescription in Canada?

No. Acetazolamide is prescription-only in Canada, and the Diamox brand is no longer sold here.

Do I need acetazolamide for Machu Picchu?

Machu Picchu is at 2,430 m, but flying into Cusco (3,400 m) is a rapid ascent that CATMAT lists as a reason to consider it. Your physician decides.

If I take acetazolamide, can I climb faster?

No. The WMS uses it alongside a gradual ascent, not instead of one, and the same symptom rules apply.

Altitude Sickness: Prevention, Acetazolamide (Diamox) and Safe Ascent