About acute mountain sickness
Acute mountain sickness (AMS) is a form of altitude sickness caused by ascent to high altitude (>2500m) before there is adequate time for acclimatisation. It is a direct result of fewer oxygen molecules inspired and a reduction in oxygen delivery to tissues.
Early, mild symptoms of AMS are similar to that of a hangover (e.g. headache, dizziness, fatigue, and nausea/vomiting) and usually occur within 36 hours at altitude.
Gradual ascent that allows sufficient time for acclimatisation is the preferred method for preventing AMS.
NHS Choices provides an overview of preventing altitude sickness which includes non-pharmacological methods.
No medicines are licensed for preventing AMS.
Acetazolamide prevents AMS by mimicking the body naturally adjusting to a change in environment.
A Cochrane review demonstrated acetazolamide reduced the risk of AMS vs placebo by a factor of 0.47 (n=2,301, 16 studies). Acetazolamide was administered one to five days prior to ascent with doses of up to 500mg/day to adults at risk of AMS.
Prescribing and dosing
- Use acetazolamide 125mg twice daily (off-label).
- Smaller doses of acetazolamide can be given by halving 250mg tablets which are scored.
- Prescribe acetazolamide one to two days before gradual ascent to high altitude and continue acetazolamide for at least two days after reaching the highest point.
- Advise people to take the second dose of acetazolamide at dinnertime rather than at bedtime as it is a diuretic.
- Trial acetazolamide for two days before ascent to high altitude because side effects can resemble the symptoms of AMS.
- Check local guidelines for processes on prescribing acetazolamide for travel.
Take into account the contraindications and cautions of acetazolamide and check if it is appropriate for the person.
Avoid acetazolamide in people with a history of anaphylaxis or other severe allergy to sulphonamide as it is a sulphonamide derivative.
Medicines not recommended
No other medicines are proven to prevent AMS.
Overall, evidence for the use of the medicines listed below to prevent AMS is inconclusive and for some, side effects are a concern:
- Iron supplements
- Magnesium citrate