Altitude sickness

Acetazolamide tablets (formerly Diamox) for altitude sickness, altitude sleep disturbance and to assist acclimatisation in unavoidable rapid height gain.

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Altitude sickness medication

All medicine supplied from UK pharmacy.

Acetazolamide 250mg

Acetazolamide 250mg (Diamox)

28-112 tablets from £14.40

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Tablets Quantity Cost
Acetazolamide 250mg 28 tablets £14.40
Acetazolamide 250mg 56 tablets £27.80
Acetazolamide 250mg 112 tablets £49.60

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Prescription issued online - small prescription fee per order.

Prescription fees

Dr Fox supplies medicine on prescription and charges a small prescription fee based on the order value of each prescription.

Prescriptions are issued by our doctors online and sent electronically to our pharmacy.

Order value Prescription fee
up to £10 £1.00
up to £20 £2.00
up to £40 £3.00
over £40 £4.00

If you have your own private or NHS paper prescription please post to our pharmacy (details).

Dr Fox prices are 25%–50% lower than other UK online clinics.

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Medical information

Written and reviewed by a team of doctors. Dr Fox is regulated by the CQC & GPhC.


Please do not order without reading and understanding this medical information.
Print information to take with you

Most trekkers and climbers do not need, and should not take acetazolamide tablets (Diamox).

  • Altitude problems are unlikely below 2,500 metres (8,000 feet).
  • Anybody can suffer from illness caused by altitude. Nobody is immune to it.
  • The best way to reduce the risk is to acclimatise adequately and be prepared to descend.

Why do we need to acclimatise to high altitude?

Air pressure gets lower as you climb higher above sea level. This means that the actual amount of oxygen in the air gets less, so every breath we take gives less oxygen to the body. Acclimatisation is the body's way of adapting to the lower levels of available oxygen. The body needs oxygen for every aspect of being alive: for the brain, liver, kidneys, and muscles to work, and even for healing cuts and sores. To adapt, the body starts to take more and deeper breaths, the heart speeds up, the body chemistry changes, and extra red blood cells are made to carry more oxygen. The kidneys make more urine (pee), to help rebalance the chemistry. Even more urine is produced if taking acetazolamide so it is especially important to keep well hydrated at altitude by drinking 3-4 litres of water per day. Acclimatisation takes a few days varying from person to person. Your body has to keep acclimatising if you keep going higher.

If your body struggles to acclimatise, you may initially develop symptoms of acute mountain sickness (AMS). This can become more severe and even life threatening, causing either fluid on the lungs - high altitude pulmonary oedema (HAPE), or fluid on the brain - high altitude cerebral oedema (HACE).

It is best to allow the body to acclimatise naturally by limiting the speed of ascent once above 2,500 metres (8,000 feet). If you have to go higher more quickly, then taking acetazolamide can help the body to acclimatise, however, taking acetazolamide can also give a false sense of security.

Many people will have a few mild symptoms of AMS, as their body adjusts. Acetazolamide can be taken to help with symptoms of AMS. It is also used along with other medicines in treating HACE.

How to acclimatise to high altitude

  • People planning to ascend to over 3,000 metres (10,000 feet) should spend at least one night at 3,000 metres before they start to go higher.
  • Above 3,000 metres each new sleep height should be between 300 metres and 500 metres (1,000-1,500 feet) higher than the previous night.
  • It is possible to go higher during the day but then descend down to sleep no more than 500 metres higher than the previous night - climb high, sleep low.
  • For every 1,000 metres (3,000 feet) of ascent, sleep at that height for 2 nights before going any higher, preferably also taking a rest day.

When choosing a group trek or tour which includes high altitude, it is important to look carefully at the itinerary and ask questions about rate of ascent, rest days, and how flexible the trip schedule will be. Kilimanjaro climbs in particular are rarely arranged within the acclimatisation guidelines, so there is an increased risk of developing problems due to the altitude. Tight schedules, pride, peer pressure, or rivalry can all contribute to higher risks of suffering from severe altitude illness.

Taking acetazolamide to help acclimatisation and to treat sleeping disorder of altitude

If unable to stick to a gradual ascent schedule, the dose of acetazolamide is 125mg (half a tablet) twice daily to help the body to acclimatise.

If significantly troubled by irregular breathing during sleep, a dose of acetazolamide of 125mg (half a tablet) can be taken two hours before sleep. This can be increased to 125mg twice daily if necessary. Acetazolamide is not a sedative.

Symptoms of mild early acute mountain sickness (AMS)

Many people will have mild symptoms of AMS when they get above 2,500 metres (8,000 feet). It can feel like a bad hangover with:

  • A headache that is not caused by dehydration (dehydration headache should settle by taking paracetamol and drinking a litre of water).
  • Fatigue and weakness.
  • Dizziness and light-headedness.
  • Difficulty sleeping and irregular breathing during sleep.
  • Loss of appetite, nausea, and vomiting.

Do not continue to ascend with these symptoms. Stop going higher until the symptoms resolve or preferably descend. Descending will usually help the symptoms to improve more quickly. Ascent can be continued after the symptoms have settled, usually after 24-72 hours.

To treat mild AMS

The severity of AMS can be assessed using the Lake Louise score and it is a good plan, when at altitude, to check symptoms against the score each day.

Mild AMS (Lake Louise score 3 to 5) should be treated as follows:

  • Stay at the height already reached or if possible go down by 300-1,000 metres.
  • Drink plenty of fluids (3-4 litres per day).
  • Avoid alcohol and sedatives.
  • Take ibuprofen for the headache.
  • Acetazolamide 250mg (one tablet) twice daily can be used, but there is still a need to wait for symptoms to settle before going any higher.
  • A doctor may also prescribe anti sickness medication or dexamethasone, if there are more severe AMS symptoms.

Emergencies requiring immediate descent

A Lake Louise score of 6 or more indicates severe AMS which can very quickly develop into HAPE or HACE. Descent is strongly recommended with a score of 6 or over.

With any symptom indicating possible HAPE or HACE, immediate descent is vital to prevent potential loss of life. Descend now! Not in an hour, or later, or in the morning. The patient should be carried or stretchered to reduce their exertion, as exercise will worsen their symptoms.

Symptoms of HAPE (fluid on the lungs) requiring immediate descent

  • Trouble breathing, breathless at rest, breathing rate taking a long time to recover after physical exertion.
  • Coughing.
  • Frothy spit, becoming bloody.
  • Lips, tongue, nail beds become blue.
  • Fast pulse and fast breathing at rest.

Symptoms of HACE (fluid on the brain) requiring immediate descent

  • Severe headache.
  • Becoming clumsy - unable to walk heel to toe in a straight line or to touch finger to nose with eyes closed.
  • Odd behaviour - unhelpful, violent, lazy, unable to think straight, or to do simple sums.
  • Non-stop vomiting.
  • Blurred or double vision.
  • Confusion.
  • Seeing haloes around objects, hearing or smelling odd things, hallucinations.
  • Reduced consciousness, coma.

If a person cannot get their breath when everybody else has regained theirs, or if a person is behaving strangely or cannot walk in a straight line, this is a medical emergency.

If any of the above symptoms of HAPE or HACE develop, treat as an emergency requiring immediate descent, even if this is during the night. The descent needs to be at least 500-1,000 metres (1,500-3,000 feet) and as soon as possible. Delay can be fatal.

Notes about HAPE and HACE

  • Can come without any warning.
  • Can develop rapidly over a period of hours.
  • Often starts at night, although can come on during the day.
  • There may be no preceding symptoms of AMS or warning.
  • Can affect people who have ascended previously to the same height without problems.
  • Can affect people who have followed the guidelines for acclimatisation.
  • Can affect people who are taking acetazolamide.
  • Can affect fit and unfit people and local people including porters.

Treatment of HAPE and HACE

Ideally any expedition/trek to altitude will have a medically trained member, who will initiate other treatment alongside descent.

HAPE is usually treated with oxygen and immediate descent by at least 1,000 metres. A drug treatment regime for HAPE may include: nifedipine, sildenafil (Viagra), and salbutamol/salmeterol inhaler.

HACE is usually treated with oxygen, and immediate descent by at least 1,000 metres. The drug treatment regime for HACE is usually acetazolamide and dexamethasone.

In all cases of AMS, HAPE and HACE, the patient should also be kept warm and given sugary drinks and food.

If there are difficulties arranging immediate descent the same effect can be provided by using a pressurised Gamow bag. This is a portable hyperbaric chamber 'bag' which increases the pressure of available oxygen in the air. The patient is put inside the bag for several hours and the pressure inside it is increased, using a foot pump. Oxygen from a cylinder can also be fed into the bag. The Gamow bag can buy time but descent as soon as possible is still recommended.

Photo of Gamow Bag

Acetazolamide for altitude sickness

Acetazolamide is a carbonic anhydrase inhibitor diuretic which increases the amount of urine produced and changes the acidity of the blood - the effect is to improve breathing.

Acetazolamide is not licensed in the UK and Europe to prevent and treat altitude sickness, although it is approved in the USA for altitude effects, and has been used worldwide for this for many years. There is a UK licence for acetazolamide to treat glaucoma, epilepsy, and as a diuretic (water tablet). Prescription for altitude, by Dr Fox is 'off label' and doctors take responsibility for the prescribing. There is no information about altitude in the manufacturer's patient information leaflet so please print out the Dr Fox altitude information leaflet and keep it with your medication.

Acetazolamide cautions and possible side effects

Acetazolamide is a 'sulpha' drug so should not be taken by anyone with a severe sulphonamide allergy. It should not be taken by anyone with severe kidney or liver disease, or abnormalities of blood chemistry.

Commonly reported side effects include: a 'tingling' feeling in the fingers, toes, and extremities; some loss of appetite; and taste disturbance. Depression may occur if taken long term.

There is more information on side effects in the manufacturer's acetazolamide patient information leaflet.

Other medicines and acetazolamide

Acetazolamide may interfere with the action of some other medications. It should not be taken at the same time as aspirin, lithium, sodium valproate, or zonisamide.

If you are taking any other routine medication then it is recommended to discuss a trip to altitude and use of acetazolamide with your regular GP prior to travel.

Extra caution is needed if considering taking acetazolamide with:

  • Epilepsy medication.
  • Heart or blood pressure medication or water tablets (diuretics).
  • Blood thinning medication.
  • Diabetic medication.
  • Any medication known to affect folic acid, eg methotrexate, trimethoprim.
  • Steroids.
  • Glaucoma medication.
  • Amphetamines.
  • Ciclosporin.

Acetazolamide dosage

Acetazolamide tablets should be swallowed without chewing, with a drink of water, before or after a meal.

Acetazolamide doses for effects of altitude
Condition Dose of acetazolamide When to stop
Acclimatisation 125mg (half a tablet) twice daily 2-3 days after highest altitude
Irregular breathing during sleep at altitude 125mg (half a tablet) 2 hours before sleep increasing to 125mg twice daily if necessary Once sleeping has improved, or once descended.
Mild Acute Mountain Sickness (AMS) 250mg (one tablet) twice daily Once symptoms have settled
Severe Acute Mountain Sickness (AMS) 250mg (one tablet) twice daily Until no longer at altitude
High altitude cerebral oedema (HACE) 250mg (one tablet) twice daily Until no longer at altitude
High altitude pulmonary oedema (HAPE) No longer recommended for treatment of HAPE


Stopping acetazolamide does not cause a rebound in symptoms. The symptoms will not be worse than they would have been if acetazolamide had not been taken in the first place.

Taking acetazolamide for early symptoms does not mean it is OK to keep ascending. Do not ascend until symptoms resolve completely, usually 24-72 hrs.

Acetazolamide does not mask serious underlying symptoms. It treats the cause, not the symptoms. If a person feels better on acetazolamide it is because their condition has improved.

Take a trial dose of half a tablet 3-4 days before travel to check for possible adverse reactions.

Treat altitude with respect. Do not imagine that a strong person can simply battle through. People who climb and hike in high places have a reputation for pushing themselves. When it comes to altitude, planning ahead, taking one's time and responding to one's own body are virtues.

Further information

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Dr Fox supplies acetazolamide tablets on prescription - you are required to answer a short medical questionnaire before ordering.

Dr Tony Steele

Authored 17 March 2010 by Dr Tony Steele
MB ChB Sheffield University 1983. Former hospital doctor and GP. GMC no. 2825328

Reviewed by Dr A. Wood, Dr C. Pugh, Dr B. Babor
Last reviewed 04 October 2022
Last updated 19 January 2024
Editorial policy


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Answer medical questions to order(altitude sickness)

Honest & accurate responses are necessary for safe medical assessment

Have you read the altitude sickness medical information?

 Please read altitude sickness medical information IN FULL.
Medical information in printable format (pdf).

Do you agree to acetazolamide being supplied to you 'off label'?

If a medicine is used for an 'off label' (unlicensed) purpose the manufacturer is not liable in case of adverse events. Doctors take the responsibility for the prescribing.

Are you allergic to acetazolamide or sulpha containing medication?

If you are not sure please check with your regular doctor before ordering acetazolamide.

Are you taking any of the following medications?

These medications interact with acetazolamide:

  • Regular aspirin
  • Lithium
  • Sodium valproate or other epilepsy medication
  • Zonisamide

If you are taking any of the following medications has your regular doctor been informed and advised that you can also take acetazolamide?

  • Heart or blood pressure medication or water tablets (diuretics)
  • Blood thinning medication
  • Diabetic medication
  • Any medication known to affect folic acid, e.g. methotrexate, trimethoprim
  • Steroids
  • Glaucoma medication
  • Amphetamines
  • Ciclosporin

Answer 'Yes' if not taking any of these medications.

Do you plan to take acetazolamide whilst pregnant?

Have you been diagnosed with kidney or liver disease or abnormalities of blood chemistry?

For organised groups: have you been advised to obtain acetazolamide by your group organisers?

For non-organised groups please answer 'Yes'.

Are you aware taking acetazolamide does NOT replace acclimatisation?

Above 3,000 metres (10,000 feet) ascend no more than 300-500 metres (1,000-1,500 feet) per day. For every 1,000 metres (3,000 feet) of ascent stop 2 nights before ascending further.

Are you aware acetazolamide is not advised where normal acclimatisation recommendations are followed?

Proper acclimatisation is the best way to deal with altitude.

Are you aware people taking acetazolamide can still get acute mountain sickness and the very serious HAPE and HACE?

See the medical information for details.

Are you aware the sleep disturbance of altitude is treated with 125mg (half a tablet) acetazolamide 2 hours before sleep increased as necessary to twice daily?

Are you aware for early mild acute mountain sickness (headache, fatigue, dizziness, loss of appetite) the acetazolamide dose is 250mg (one tablet) twice daily?

Taking acetazolamide will help with these symptoms. The important thing is not to ascend until the symptoms resolve.

Are you aware in situations where forced ascent cannot be avoided the dose of acetazolamide is 125mg (half a tablet) twice daily?

Ascent of more than 500 metres (1,500 feet) a day above 3,000 metres (10,000 feet) is dangerous with or without acetazolamide.

Are you aware that confusion, loss of coordination, and extreme shortness of breath are serious life threatening symptoms requiring immediate descent and medical treatment?

Medical treatment may include oxygen, acetazolamide (one 250mg tablet twice daily), steroids, nifedipine, sildenafil, and salbutamol.

Do you have any further medical information or questions?

Is there anything you do not understand or do you need further help?

Do the following apply to you?

  • The medication is for my own use and I will not share with anyone else
  • I will read the patient information leaflet supplied with the medication
  • I am over 18 and I agree to identity verification checks
  • I have completed this questionnaire myself and fully understand all the information
  • My responses are honest and accurate, and I understand that this is necessary for a safe medical assessment
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