Altitude Sickness Medications: When to Take Them, Where You Need Them

Acetazolamide is the only FDA-approved AMS prophylaxis medication, yet most travellers flying into high-altitude cities like Cusco or La Paz arrive without it.

Altitude sickness and your medications: what actually changes when you travel

Acetazolamide is the only FDA-approved medication for acute mountain sickness prophylaxis, yet the majority of travellers flying directly into high-altitude cities arrive without it. That gap is where altitude sickness does its damage.^[1]

This article covers the three medication options recommended by the 2024 Wilderness Medical Society (WMS) guidelines, the destinations where you genuinely need them, and why your flight itself may be part of the problem before you even arrive.

Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Altitude sickness can progress to life-threatening conditions. Always consult your doctor or a travel medicine clinic before travelling to high-altitude destinations, particularly before starting any medication for prophylaxis.

The three medications you need to know

Acetazolamide (brand name Diamox) is the first-line choice. It works by accelerating acclimatisation: it induces a bicarbonate diuresis, stimulates ventilation, and improves sleep-breathing patterns. Critically, it does not mask symptoms, if you worsen on acetazolamide, you are genuinely worsening.^[1]

The standard prevention dose is 125 mg twice daily, started 24 hours before ascent and continued for two days at maximum altitude. The 2024 WMS guidelines note that 250 mg twice daily may be considered for higher-risk ascent profiles, such as a compressed itinerary with little acclimatisation time built in. The main contraindication is a severe sulfonamide allergy.^[2]

Dexamethasone is more effective than acetazolamide at rapidly relieving symptoms of moderate to severe AMS, but with an important catch: it does not improve acclimatisation itself.^[4] Using dexamethasone to feel well enough to keep ascending is dangerous. The 2024 WMS guidelines recommend it as an alternative to acetazolamide for adult travellers at moderate or high risk, or as an emergency carry for treatment, not as the primary prevention drug for most itineraries.^[3]

Ibuprofen is the over-the-counter option. Randomised controlled trials show that 600 mg three times daily decreased AMS symptoms compared to placebo. However, studies attribute this to ibuprofen's analgesic effect rather than facilitating acclimatisation.^[5] The WMS guidelines position it as the choice for travellers who cannot take acetazolamide or dexamethasone. It does not have the acclimatisation-accelerating effect of acetazolamide, and it has not been directly compared to dexamethasone in trials.^[2]

Destinations where the medication conversation is not optional

Cusco, Peru sits at 3,399 m (11,150 ft). Most travellers fly in from Lima at sea level, placing them at high risk for AMS within hours of arrival. The 2024 WMS guidelines place any ascent that results in sleeping above 3,500 m on day one in the highest risk category, where acetazolamide prophylaxis is strongly recommended.^[2]

La Paz, Bolivia has an international airport at 4,061 m (13,323 ft), making it the highest commercial airport in the world. Travellers continuing on to Lake Titicaca (3,812 m), the Uyuni salt flats (3,656 m), or Potosí (4,090 m) face sustained high-altitude exposure that compounds initial acclimatisation challenges. Pre-travel chemoprophylaxis is standard for these itineraries.

Lhasa, Tibet sits at 3,650 m (11,975 ft) and is accessible only by flights that ascend rapidly. Pre-travel chemoprophylaxis is the rule rather than the exception for Lhasa itineraries. Travellers with any personal history of high-altitude cerebral oedema (HACE) or high-altitude pulmonary oedema (HAPE) should seek specialist consultation before planning any Tibet plateau itinerary.

Nepal's trekking routes span a wide range. Kathmandu itself sits at 1,400 m and poses minimal risk on arrival. The Everest Base Camp route reaches 5,364 m at base camp and 5,545 m at Kala Patthar. The Annapurna Circuit crosses Thorong La pass at 5,416 m. Both involve sustained sleeping altitudes above 4,000 m for multiple consecutive nights.

Studies report that approximately 40 to 60% of trekkers experience AMS symptoms during the Everest Base Camp trek.^[6] Kilimanjaro (5,895 m) in Tanzania also belongs in this category: its most common trekking itineraries gain sleeping altitude too fast to avoid high-risk classification under WMS criteria.

The flight itself: a risk most travellers overlook

Commercial aircraft cabins are pressurised to the equivalent of 6,500 to 8,000 feet above sea level, not to sea level. A simulated flight study of 502 participants found that 7.4% developed acute mountain sickness symptoms from cabin pressure changes alone. That figure matters: your body may already be mildly hypoxic before you step off the plane in Cusco.

Passengers with pre-existing cardiac or pulmonary conditions face a specific risk: the cabin environment at 6,500 to 8,000 ft can cause an 8 to 10% reduction in oxygen saturation, which can impair cognitive function and physical endurance even before landing.^[8] Altitude can also affect blood pressure and how antihypertensives perform, as explained in our guide on blood pressure medication at high altitude.

The practical consequence is this: if you are flying directly from a low-altitude departure point into a high-altitude destination, your acclimatisation clock starts ticking in the aircraft, not at the airport gate on arrival. Starting acetazolamide 24 hours before your flight, as the WMS guidelines suggest for rapid ascent scenarios, accounts for this.^[7]

Getting acetazolamide before you travel: prescription rules

Acetazolamide is prescription-only in the United States, the United Kingdom, Canada, and Australia. It is not a controlled substance under any national drug schedule or international convention, so no special import permit is required at transit airports. The documentation to carry is straightforward: the original pharmacy box, a copy of your prescription, and a doctor's letter naming the drug and the dose.

Nepal lists acetazolamide on its National List of Essential Medicines, but the listed tablet is 250 mg. The current standard prevention dose of 125 mg twice daily means a locally purchased tablet would require splitting. Supply consistency at altitude pharmacies is not guaranteed. Bring your supply from home.

Dexamethasone and ibuprofen are also not controlled substances. Dexamethasone is a corticosteroid, not a scheduled anabolic steroid, and requires no special customs declaration beyond carrying it in its labelled packaging.

The rule that medication cannot replace

Physical fitness does not protect against altitude sickness. A highly conditioned athlete ascending rapidly is at the same or greater risk as an unfit traveller who ascends slowly. The 2024 WMS guidelines set a maximum sleeping altitude increase of 500 m per night above 3,000 m, with a rest day every 3 to 4 days. Acetazolamide reduces risk at a given ascent rate; it does not make a reckless ascent rate safe.^[3]

If symptoms worsen at the same altitude despite treatment, descent is mandatory. HACE and HAPE are medical emergencies requiring immediate descent of at least 500 to 1,000 m, regardless of itinerary schedule or proximity to a destination.

Carrying your medical records at altitude

Remote high-altitude locations such as Everest Base Camp trekking routes or the Bolivian Altiplano have limited healthcare infrastructure. An emergency at altitude is far harder to manage when the treating provider cannot access your medical history, allergy list, or current medications.

Keeping your medication import rules documentation alongside a current medication list in Nomedic means any provider, whether at a Himalayan Rescue Association aid post or a hospital in La Paz, can access the information they need. Your IPS record also documents any sulfa allergy that would make acetazolamide unsafe, the kind of fact that matters most in a crisis at 4,500 m.

Before any high-altitude trip, verify that your travel insurance covers medical evacuation at the specific altitude ceiling of your destination. Helicopter evacuation from villages near Everest Base Camp typically costs $3,000 to $6,000 (~€2,800 to ~€5,600), with costs reaching $10,000 (~€9,300) in the most difficult conditions. A policy without a high-altitude clause can leave that bill entirely unpaid.

Frequently asked questions

Is acetazolamide a controlled substance when travelling internationally?

No. Acetazolamide is prescription-only but is not scheduled as a controlled substance under any national drug schedule or international convention. Transit airports in the UAE, Qatar, and Singapore target narcotic and psychotropic medicines through their permit systems, and acetazolamide falls into neither category. Carry it in the original labelled packaging with a copy of your prescription and a doctor's letter.

How early before flying to altitude should I start acetazolamide?

The 2024 Wilderness Medical Society guidelines recommend starting acetazolamide and dexamethasone the day before ascent, though both still have beneficial effects if started on the day of ascent. For itineraries involving rapid ascent to high-altitude cities with international airports, such as Lhasa or Cusco, starting 24 hours before your flight is the more conservative approach recommended by travel medicine clinicians.

Can I get altitude sickness on a plane before I even land?

Yes. Commercial aircraft cabins are pressurised to the equivalent of 6,500 to 8,000 feet above sea level. A 2007 New England Journal of Medicine study of 502 participants found that 7.4% developed acute mountain sickness symptoms from cabin pressure changes alone. If you are flying directly into a high-altitude destination such as Cusco or La Paz, your body begins adapting in the aircraft, not on arrival.

Is Diamox available over the counter in Nepal or Peru?

Nepal lists acetazolamide on its National List of Essential Medicines, and it is available in Kathmandu pharmacies, but the listed tablet is 250 mg rather than the 125 mg standard prevention dose. The CDC advises using medication brought from home rather than relying on local supply at altitude. In Peru, acetazolamide requires a prescription and local stock quality is inconsistent at high-elevation pharmacies.

Does taking Diamox mean I can ascend faster on trek?

No. Acetazolamide reduces AMS risk at a given ascent rate; it does not make a faster rate safe. The 2024 WMS guidelines still require a maximum of 500 m of sleeping altitude gain per night above 3,000 m, regardless of whether you are taking medication. Using it as a licence to skip acclimatisation days is the most common misuse.

Is ibuprofen as effective as Diamox for preventing altitude sickness?

Randomised controlled trials show 600 mg of ibuprofen three times daily reduces AMS incidence compared to placebo, but studies comparing it directly to acetazolamide favour acetazolamide for rapid ascent. The 2024 WMS guidelines position ibuprofen as the choice for travellers who cannot take acetazolamide or dexamethasone due to allergy or intolerance, not as an equivalent alternative for high-risk itineraries.