Altitude sickness tablets & advice for mountaineers and expeditions
Trekking Kilimanjaro or Everest Base Camp, walking the Inca Trail, or heading out on a high-altitude expedition? Altitude sickness can affect anyone above about 2,500 m — regardless of fitness. We provide pre-expedition consultations with acclimatisation advice, an emergency plan, and altitude sickness tablets (acetazolamide / Diamox) where appropriate — plus any travel vaccines for the same trip.
Medically reviewed by Muhammad Adnan, Superintendent Pharmacist (GPhC 2073652) · based on NaTHNaC (TravelHealthPro) and NHS guidance · last reviewed June 2026.
What is altitude sickness?
At altitude there is less oxygen in the air, and your body needs time to adjust — a process called acclimatisation. Altitude sickness (acute mountain sickness, or AMS) happens when you ascend faster than your body can adapt. It can start at around 2,500 m and becomes more likely the higher and faster you go. It is not a sign of being unfit — strong, experienced climbers get it too — and you cannot reliably predict it from previous trips. Most cases are mild and settle with rest, but altitude sickness can progress to two dangerous conditions if ignored.
Know the three forms — and the danger signs
AMS — mild
Like a hangover: headache plus nausea, tiredness, dizziness, poor appetite or disturbed sleep. Stop ascending, rest and hydrate until it settles.
HACE — emergency
Fluid on the brain: an unsteady, drunken-looking walk, confusion, or drowsiness. Descend at once and get urgent help; dexamethasone may be used.
HAPE — emergency
Fluid in the lungs: breathlessness at rest, a cough, tightness and exhaustion. Descend at once, get oxygen and urgent help; nifedipine may be used.
The golden rule: never go higher with any symptoms of altitude sickness, and if symptoms are severe or getting worse, descend. Descent is the single most effective treatment — medication and oxygen buy time, but coming down is what saves lives.
The acclimatisation rules that prevent it
Gradual ascent is the foundation of staying well at altitude. The widely used rules of thumb are:
- Above 3,000 m, try not to increase your sleeping altitude by more than 300–500 m per day.
- Take a rest day every 3–4 days, or roughly every 1,000 m gained.
- “Climb high, sleep low” — it is fine to walk higher during the day, but sleep at a lower altitude.
- Stay well hydrated, eat enough, and avoid alcohol and sedatives, especially in the first days.
- Build a flexible itinerary with spare days, so you can rest or descend without losing the whole trip.
- Know the symptoms, watch your group, and agree a plan for what you'll do if someone is affected.
Altitude sickness tablets: acetazolamide (Diamox)
Acetazolamide, better known by the brand name Diamox, is the main tablet used to prevent and ease altitude sickness. It gently stimulates your breathing, which helps your body acclimatise faster to the thinner air. Taken for prevention it reduces the likelihood of AMS; it can also help treat mild symptoms.
How it's usually taken
- A common preventive dose is 125 mg twice a day.
- Started about 24 hours (1–2 days) before you begin ascending.
- Continued for 2–3 days after reaching your highest sleeping altitude (or while still climbing).
- The exact dose and plan are decided at your consultation.
Good to know
- Common, harmless effects: tingling in fingers/toes, passing urine more often, and fizzy drinks tasting odd.
- It contains a sulfonamide — tell us about any sulfa allergy.
- It's an aid to acclimatisation, not a reason to ascend faster.
- For severe illness, dexamethasone (HACE) and nifedipine (HAPE) may be carried on expeditions — alongside descent and oxygen.
Acetazolamide is a prescription-only medicine. We assess your itinerary, ascent profile and medical history before prescribing, and explain exactly how and when to take it.
Treks and expeditions where it matters
If your trip takes you above ~2,500 m, altitude advice is worth having. Common examples:
- Africa: Kilimanjaro (5,895 m), Mount Kenya, the Atlas Mountains / Toubkal.
- Asia: Everest Base Camp, Annapurna and the Nepal trails, Ladakh and Leh, Tibet.
- South America: the Inca Trail and Machu Picchu, Cusco, the Andes, Aconcagua, Bolivia's La Paz and Lake Titicaca.
- Expeditions & skiing: high-altitude climbing trips, ski-mountaineering and high resorts.
Altitude FAQ
Altitude sickness — your questions answered
The main altitude sickness tablet is acetazolamide (brand name Diamox). It works by mildly acidifying the blood, which stimulates breathing and helps your body acclimatise faster to the lower oxygen at altitude. Used for prevention it reduces the chance of acute mountain sickness (AMS) and eases symptoms. It is an aid to acclimatisation, not a licence to ascend faster — sensible, gradual ascent is still the most important thing. It is a prescription medicine, so we assess your itinerary and medical history first.
For prevention, a common regimen is acetazolamide 125 mg twice a day, started about 24 hours (1–2 days) before you begin ascending and continued for 2–3 days after reaching your highest sleeping altitude (or while you keep climbing). The exact dose and timing are decided at your consultation. Common, harmless side effects include tingling in the fingers and toes, needing to pass urine more often, and fizzy drinks tasting flat or odd. It contains a sulfonamide, so tell us about any sulfa allergy.
Acute mountain sickness can begin at around 2,500 m (about 8,000 ft) and becomes more likely the higher and faster you go. Some people feel effects a little lower. It is not related to fitness — very fit people get it too — and how you coped at altitude before is only a rough guide. Anyone ascending to high altitude should know the symptoms and the acclimatisation rules.
Mild AMS feels like a hangover: headache plus nausea, tiredness, dizziness, poor appetite or poor sleep. The danger signs that mean it has become an emergency are: an unsteady, drunken-looking walk, confusion or drowsiness (signs of HACE — fluid on the brain), or breathlessness at rest, a persistent cough or very low energy (signs of HAPE — fluid in the lungs). HACE and HAPE are life-threatening: the response is immediate descent and urgent medical help. Never ascend with any symptoms of altitude sickness.
Any trip that takes you above ~2,500 m. Popular ones include Kilimanjaro, Everest Base Camp and the Annapurna trails in Nepal, the Inca Trail and Machu Picchu, Cusco and the Andes, Aconcagua, Ladakh and Leh, the Atlas Mountains (Toubkal), Bolivia's La Paz and Lake Titicaca, and high-altitude skiing or climbing expeditions. We tailor advice to your exact route and ascent profile.
No — and this matters. Tablets reduce risk but do not make a fast ascent safe. The cornerstone is gradual ascent: above 3,000 m, try not to increase your sleeping altitude by more than 300–500 m per day, take a rest day every 3–4 days or every ~1,000 m, 'climb high, sleep low', stay well hydrated and avoid alcohol and sedatives. If you develop symptoms, stop ascending; if they are severe or worsening, descend.
Yes. At Altrincham Travel Clinic in Timperley — between Altrincham and Manchester, serving Trafford, Stockport, Cheshire and the wider area — we run pre-expedition consultations: we review your route and health, give acclimatisation and emergency-plan advice, and can prescribe altitude sickness tablets (acetazolamide) where appropriate. We can sort any travel vaccines for the same trip at the same visit. Book ahead of your departure.
Important: This page is general information based on NaTHNaC (TravelHealthPro) and NHS guidance and is not a substitute for individual medical advice. Altitude medication is prescribed only after an assessment. Descent and urgent medical help are the priority for severe altitude illness (HACE/HAPE). Sources: TravelHealthPro altitude illness factsheet · NHS altitude sickness.
Planning a trek or expedition?
Book a pre-expedition consultation at our Timperley clinic, near Manchester. We'll review your route, give acclimatisation and emergency-plan advice, prescribe altitude sickness tablets where appropriate, and sort any travel vaccines for the same trip — for travellers across Manchester, Trafford, Stockport and Cheshire.