How to Prevent Altitude Sickness: Ascent Rules and Acclimatisation
The practical rules for going high safely: how fast to climb, when to rest, what helps and what doesn't, and when tablets are worth considering.
Part of the Access Doctor Altitude Sickness Guide.
Key fact: Above 3,000m, don't raise your sleeping height by more than 500m a day, and take a rest day every 3 to 4 days. A slow ascent prevents more altitude sickness than any tablet.
Altitude sickness is caused by going up too quickly, so the way you plan your trip matters more than anything you take. This guide sets out the rules used by UK travel health services. For symptoms and treatment, see our guide to altitude sickness.
Get Altitude Sickness Tablets Online
Tell us about your trip and your health. A UK pharmacist prescriber checks whether preventive treatment is suitable for you. Approved before 3pm, Mon–Fri? Dispatched the same day.
Start Altitude Sickness Consultation →The Rules of Safe Ascent
1
Avoid a big first jump
Where you can, don’t go from below 1,200m to above 3,500m in one day. If you fly or drive straight to a high place, plan 2 to 3 easy days there before going higher.
2
500m a day above 3,000m
Raise the height you sleep at by no more than 500m each day. What counts is where you sleep, not how high you walk during the day.
3
A rest day every 3 to 4 days
Spend a second night at the same height. Short walks higher during the day are fine: “climb high, sleep low”.
4
Never go higher with symptoms
If you have a headache with nausea, dizziness or unusual tiredness, stay where you are until it clears.
5
Go down if you’re getting worse
Descending even 500m usually brings quick relief.
Check your itinerary before you book. Some commercial treks go up faster than these rules, because shorter trips cost less. Choosing a longer route, with extra acclimatisation days, is the single best thing you can do.
How Acclimatisation Works
Within hours of arriving at altitude you breathe faster and your heart beats faster. Over the next few days your kidneys adjust your blood chemistry so you can keep breathing harder, and over weeks you make more red blood cells. Acclimatising to a given height usually takes a few days, and the benefit fades once you return to low altitude.
Waking at night, vivid dreams and pauses in breathing during sleep are common at altitude and usually harmless.
What Helps, and What Doesn’t
| Helps | Doesn’t help, or isn’t proven |
|---|---|
| A slow ascent with rest days | Being very fit |
| Drinking enough to keep your urine pale | Drinking far more water than you need |
| Eating regularly, with plenty of carbohydrate | Coca leaves or coca tea, and herbal remedies such as ginkgo |
| Avoiding alcohol and sleeping tablets | Canned oxygen sold in tourist shops |
| Taking it easy for the first 1 to 2 days | Painkillers taken to “push on” through symptoms |
Do You Need Tablets?
Tablets aren’t needed for low-risk trips. They’re worth considering when:
- You’ve had altitude sickness before
- You’re flying or driving straight to above 2,800m, such as Cusco, La Paz or Lhasa
- Your itinerary goes up faster than 500m a day above 3,000m, or has few rest days, as on many Kilimanjaro routes
The medicine used is acetazolamide, which helps your body acclimatise faster. See acetazolamide for altitude sickness for the dose and side effects, and altitude sickness by destination for popular trips. If you’ve ever had HACE or HAPE, see a travel clinic or specialist before you go.
If Symptoms Start
The three golden rules: 1. If you feel unwell at altitude, assume it’s altitude sickness until proven otherwise. 2. Never go higher to sleep while you have symptoms. 3. If you’re getting worse, go down straight away.
For mild symptoms, rest at the same height, drink fluids and take paracetamol or ibuprofen for headache (see altitude sickness medication compared). Most people improve within 1 to 2 days. Tell your guide or companions how you feel: people with altitude sickness often play it down, and those with severe illness may not realise how unwell they are.
Checklist Before You Go
- Itinerary checked against the 500m rule, with rest days
- Travel clinic or pharmacist appointment 4 to 6 weeks before, for vaccines and advice
- Insurance that covers your highest altitude and emergency evacuation
- Preventive tablets, if advised, with a trial dose taken at home
- Sun protection, warm layers and a basic first aid kit
Need Tablets for Your Trip?
Tell us about your trip and your health. A UK pharmacist prescriber checks whether preventive treatment is suitable for you. Approved before 3pm, Mon–Fri? Dispatched the same day.
Start Altitude Sickness Consultation →Frequently Asked Questions
How can I avoid altitude sickness?
Go up slowly. Above 3,000m, sleep no more than 500m higher each night and take a rest day every 3 to 4 days. Avoid alcohol, drink enough fluids, and consider preventive tablets if your risk is moderate or high.
How long does it take to acclimatise to altitude?
Acclimatising to a given height usually takes a few days. You need to acclimatise again each time you go significantly higher, and the benefit fades once you return to low altitude.
Does drinking water prevent altitude sickness?
Staying hydrated helps you feel better, because dehydration causes similar symptoms, but drinking extra water doesn't prevent altitude sickness. Drink enough to keep your urine pale.
Do coca leaves help with altitude sickness?
There's no good evidence that coca leaves or coca tea prevent or treat altitude sickness. Coca can also give a positive result on a drug test.
What does 'climb high, sleep low' mean?
It means you can walk to a higher point during the day, as long as you come back down to sleep. Your sleeping height is what matters for acclimatisation.
Should I take altitude sickness tablets just in case?
Not for low-risk trips. Tablets are worth considering if you've had altitude sickness before, are going straight to above 2,800m, or have a fast ascent. A prescriber can advise from your itinerary.
References
- NaTHNaC, TravelHealthPro. Altitude illness. travelhealthpro.org.uk
- Fit for Travel (Public Health Scotland). Altitude and travel. fitfortravel.nhs.uk
- Luks AM, et al. Wilderness & Environmental Medicine. Wilderness Medical Society clinical practice guidelines for the prevention, diagnosis and treatment of acute altitude illness: 2024 update. journals.sagepub.com
- NHS. Altitude sickness. nhs.uk
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Prescription medicines require a consultation before they can be dispensed. Always consult a qualified healthcare professional for diagnosis and treatment. In a medical emergency, call 999.