For altitude sickness prevention, acetazolamide (Diamox) is taken as 125 mg twice daily, starting the day before ascent and continuing for 48 hours after reaching your highest sleeping altitude. It works by speeding up acclimatization itself rather than masking symptoms, which makes it the only prophylactic the Wilderness Medical Society recommends for most travelers. It’s prescription-only in the US.
Altitude sickness doesn’t care how fit you are. It hits marathoners and couch dwellers at the same rates, it starts as low as 8,000 feet, and the itineraries that trigger it are ordinary ones: flying into Cusco (11,150 ft), driving to a Breckenridge condo (9,600 ft), or landing in La Paz (11,900 ft). If your trip sleeps above 8,000 feet, this is the medication conversation to have before you go.
What altitude sickness actually is
At altitude there’s less oxygen per breath, and your body’s fix (breathing faster) has a side effect: it blows off carbon dioxide, making your blood more alkaline, which then suppresses the very breathing increase you need. That stalemate is why you feel fine on arrival and lousy at 2 a.m.: headache, nausea, exhaustion, and sleep that keeps jolting awake. That’s acute mountain sickness (AMS), and it hits roughly 25% of people sleeping above 8,000 feet and more than half of unacclimatized travelers above 11,500.
Acetazolamide breaks the stalemate. It makes your kidneys dump bicarbonate, re-acidifying the blood, which lets your breathing stay high and your oxygen levels climb. You still acclimatize; you just do it in hours instead of days.
Dosing and timing
| Use | Standard adult dosing |
|---|---|
| Prevention | 125 mg twice daily, starting 24 hours before ascent |
| Continue until | 48 hours at your highest sleeping elevation |
| Treatment of symptoms | 250 mg twice daily, plus stop ascending |
| Kids | Weight-based, pediatric provider territory |
Two timing notes that save trips. First, the day-before start matters; starting on the mountain works but gives up the head start, and the whole point is being ahead of the 2 a.m. headache on night one. Second, acetazolamide is a mild diuretic, so take the second dose with dinner rather than bedtime unless you enjoy 3 a.m. bathroom trips at altitude, where sleep is already fragile.
What it feels like to take
The signature side effect is tingling in fingers, toes, and lips (paresthesia), harmless and common enough that mountain guides treat it as proof you took your dose. The other famous one: carbonated drinks taste flat and metallic, a party trick of the drug’s effect on the enzyme in your taste buds. Beer at the ski lodge will taste off. Plan emotionally.
The real screening questions: acetazolamide is a sulfonamide, so severe sulfa reactions need a provider conversation (true cross-reactivity is rare, but the history matters). It’s avoided with significant kidney or liver disease, and it stacks with other diuretics.
What it doesn’t replace
Acetazolamide supports acclimatization; it doesn’t license a reckless ascent profile. The non-negotiables still apply: once above 9,000 feet, raise your sleeping elevation no more than about 1,500 feet a night, hydrate, go easy on alcohol the first two nights, and treat a worsening headache with confusion or breathlessness at rest as an emergency that means descend now. Severe altitude illness (HACE, HAPE) is rare on ski trips but it’s a mountain rescue situation, not a medication adjustment.
Getting it before your trip
Acetazolamide is prescription-only. The telehealth fit is natural here too: complete OZIO’s online assessment with your itinerary and elevations, a licensed physician reviews it, and acetazolamide 250 mg ships before departure (the 250 mg tablets split cleanly for the 125 mg prevention dose; score lines exist for this). For trips that combine altitude with other exposures, a Peru itinerary with Cusco plus the Amazon say, the same review can cover the rest of the travel kit: ondansetron for nausea, azithromycin for GI backup, and malaria prophylaxis where the lowlands require it.
FAQ
How fast does acetazolamide work? Its breathing effect starts within hours of the first dose, which is why even a day-before start provides real protection for night one at elevation.
Does ibuprofen work instead? Ibuprofen treats the altitude headache and has modest evidence for prevention, but it masks symptoms rather than speeding acclimatization. Fine as a companion, not a substitute.
Can I just drink coca tea in Peru? Enjoy it as culture, not as prophylaxis. The evidence for meaningful AMS prevention is thin, and it can complicate drug tests for weeks after.
Do I need it for Denver? At 5,280 feet, almost nobody does. The usual threshold for considering prophylaxis is sleeping above 8,000 feet, or a history of AMS at lower elevations. Denver to Summit County the same day is the move that catches people.
This article is for informational purposes only and is not medical advice. Acetazolamide requires evaluation by a licensed healthcare provider. Severe altitude symptoms (confusion, breathlessness at rest, inability to walk straight) are an emergency: descend and seek care.
