Acetazolamide

Helping to prevent acute mountain sickness on a fast ascent to high altitude.

It is also used to treat mild altitude sickness that has already started, alongside stopping the ascent.

Acetazolamide blocks an enzyme called carbonic anhydrase in the kidney. You lose bicarbonate in the urine, the blood becomes slightly more acidic, and that acidity drives you to breathe more deeply.

That is the same adjustment the body makes on its own over several days. The tablet does it faster.

Adults, 18 and over, going high enough and fast enough for it to be worth taking. Preventative medicine is not needed for a gradual ascent and it is not a substitute for one.

Not suitable if you have ever had a severe reaction to a sulfonamide medicine, in pregnancy, or if you have significant kidney or liver problems.

You tell us where you are going and how high you plan to sleep, we read your answers and decide whether this is right for that trip, and if it is we dispense it and post it to you.

If your ascent is gradual enough not to need it, we will tell you that instead of selling it to you.

Tracked: Tracked delivery, arrives in two working days - Free

When it arrives: Every order is sent tracked. Where this page says in post today, it goes out today. Where it says in post tomorrow, it goes out the next working day. On the rare occasions a medicine is unavailable, we will contact you to talk about timing and alternatives.

We dispense clinically equivalent generic medications where available.

Acetazolamide from £18.99 per pack.

Very common and expected: passing more urine, and pins and needles in the fingers, toes and around the mouth.

Also common: a metallic or flat taste, particularly with fizzy drinks, feeling sick, headache and tiredness.

Less common: dizziness, kidney stones, changes to the blood counts, ringing in the ears.

Rare: severe skin reactions, allergic reactions, low mood, confusion.

The tingling and the extra urine are the medicine working rather than a problem, which is exactly why a trial dose at home is worth taking.

A history of hypersensitivity to a sulfonamide medicine.

Pregnancy, particularly the first trimester.

Significant kidney impairment, because of the risk of metabolic acidosis, and significant liver impairment.

Adrenal gland insufficiency, low potassium, low sodium, or an acid imbalance in the blood.

Long-term use in chronic closed-angle glaucoma.

Aspirin in high doses raises the risk of acidosis.

It can raise lithium clearance and alter the effect of some diabetes medicines, phenytoin and ciclosporin.

Tell us everything you take, including anything you buy over the counter.

Pregnancy: avoid, particularly in the first trimester. Both the manufacturer and the UK travel medicine guidance say so. There is also little safety data on travel to high altitude in pregnancy at all, so this is a conversation to have with a doctor before booking, not after.

Breastfeeding: the amount that reaches the baby is too small to be harmful.

Acetazolamide is licensed in the UK for glaucoma and epilepsy. Using it to prevent altitude sickness is off-label, which means outside the licence. It is what UK travel medicine recommends and it is what we prescribe, and we tell you because you should know what you are taking and why.

How to take it: 125mg twice a day, which is half a 250mg tablet morning and evening. Start it the day before you begin going up, and carry on for at least two days after you reach your highest point. If you are climbing to a summit and coming straight back down, and you feel well, stop when you start descending.

Take a trial dose for a day or two before you travel, so you find out how it suits you at home rather than on a mountain.

The tablets do not replace going up slowly. Above 3,000m, do not increase your sleeping altitude by more than 500m a day and take a rest day every three or four days. Never go higher to sleep while you have symptoms of altitude illness, and always go down if symptoms get worse or are severe.

If you get a rash of any kind while taking it, stop and tell a doctor. Acetazolamide is related to the sulfonamides and a rash can be the first sign of a serious skin reaction.