Jasins Pharmacy
Key Takeaways
- Acetazolamide (Diamox) is the most clinically proven and widely prescribed altitude sickness tablet for the prevention and treatment of AMS.
- Altitude sickness typically begins above 2,500 metres and can affect anyone regardless of age, fitness level, or previous altitude experience.
- Acetazolamide requires a prescription in the UK and is not suitable for everyone — a consultation with a pharmacist or doctor is essential before starting the medication.
- Medication is not a substitute for proper acclimatisation — the golden rule of “climb high, sleep low” and allowing adequate rest days remains essential.
- High-altitude cerebral oedema (HACE) and high-altitude pulmonary oedema (HAPE) are life-threatening complications that require immediate descent and emergency treatment.
- Jasins Pharmacy in Hounslow offers private altitude sickness consultations and prescriptions — no GP referral required, with same-week appointments available.
The Effects of Altitude Sickness
Altitude sickness — medically known as acute mountain sickness (AMS) — occurs when the body is exposed to reduced levels of oxygen at high altitude and fails to acclimatise quickly enough. At high altitude, the atmospheric pressure is lower, which means each breath contains fewer oxygen molecules than at sea level. The body responds to this reduced oxygen availability by breathing faster and producing more red blood cells, but these adaptations take time — and if ascent is too rapid, the body cannot keep pace with the physiological demands being placed on it.
AMS typically begins above 2,500 metres and becomes increasingly common above 3,500 metres. It affects people of all ages, fitness levels, and backgrounds — elite athletes are just as susceptible as recreational hikers, and previous successful ascents offer no guarantee of immunity on a future trip. Individual susceptibility varies considerably and cannot be reliably predicted in advance, which is one of the reasons why pharmacological prevention with acetazolamide is such an important option for travellers undertaking rapid or ambitious ascents.
Popular high-altitude destinations visited by UK travellers include the Everest Base Camp trek in Nepal (5,364 metres), Kilimanjaro in Tanzania (5,895 metres), Machu Picchu and the Inca Trail in Peru (up to 4,200 metres), the Bolivian altiplano (3,600 metres and above), Lhasa in Tibet (3,650 metres), and Cusco in Peru (3,400 metres). Each of these destinations carries a genuine risk of altitude sickness, particularly for travellers who fly directly into high-altitude airports or begin trekking without adequate acclimatisation days.
Recognising the Symptoms of Altitude Sickness
Understanding the symptoms of altitude sickness — and knowing when they have progressed to something more serious — is one of the most important pieces of knowledge any high-altitude traveller can have. AMS exists on a spectrum, from mild and manageable to severe and life-threatening, and knowing where on that spectrum you or a fellow traveller sits is essential for making the right decisions in the mountains.
Mild to Moderate AMS
The symptoms of mild to moderate AMS typically appear within six to twelve hours of arriving at a new altitude. They include a headache, which is the cardinal symptom and is usually described as a dull, throbbing ache that worsens with exertion, fatigue and general tiredness that is disproportionate to the level of activity, loss of appetite and nausea, dizziness or lightheadedness, and difficulty sleeping. At this stage, the correct response is to stop ascending, rest at the current altitude, stay well hydrated, and allow the body time to acclimatise. Acetazolamide can be used at this stage to accelerate recovery.
Severe AMS and Life-Threatening Complications
If mild AMS is not managed properly or ascent continues despite symptoms, the condition can progress to high-altitude cerebral oedema (HACE) or high-altitude pulmonary oedema (HAPE) — both of which are medical emergencies that can be fatal within hours if not treated. HACE involves swelling of the brain and manifests as severe headache unresponsive to painkillers, loss of coordination (ataxia), confusion and irrational behaviour, and eventually loss of consciousness. HAPE involves fluid accumulation in the lungs and presents as breathlessness at rest, a persistent dry cough that may produce pink frothy sputum, extreme fatigue, and cyanosis (bluish discolouration of the lips or fingernails). The only definitive treatment for both HACE and HAPE is immediate descent — no amount of medication replaces getting lower.
Acetazolamide (Diamox) — The Gold Standard
Acetazolamide is a carbonic anhydrase inhibitor — a class of drugs that were originally developed as diuretics but were found to have a powerful acclimatisation-accelerating effect at altitude. By inhibiting the carbonic anhydrase enzyme, acetazolamide causes a mild metabolic acidosis in the blood, which stimulates the respiratory centre in the brain to drive faster and deeper breathing. This increased ventilation raises blood oxygen levels more quickly than the body would achieve unaided, effectively speeding up the natural acclimatisation process.
Multiple randomised controlled trials have confirmed that acetazolamide significantly reduces the incidence of AMS when taken as a prophylactic medication before and during ascent. The Cochrane review found acetazolamide effective for AMS prevention across a range of doses; both 250mg and 500mg daily regimens show benefit, and the 125mg twice daily dose is also supported and associated with fewer side effects. At Jasins Pharmacy in Hounslow, our pharmacists can discuss the correct dosing protocol for your specific itinerary, the timing of when to start the medication, and whether acetazolamide is the right choice for your trip.
Standard Prophylactic Dose
The standard adult prophylactic dose of acetazolamide for AMS prevention is 125mg to 250mg twice daily, starting one to two days before ascending to altitude and continuing for at least two days after reaching the highest point of your trek. Some guidelines and prescribers favour the lower 125mg twice daily dose as it has been shown to be effective while producing fewer side effects. The dose and timing should always be confirmed with a prescribing pharmacist or doctor based on your specific circumstances.
Treatment Dose
If AMS develops despite prophylaxis, or if acetazolamide was not started preventatively, it can also be used as a treatment at a dose of 250mg twice daily. However, it is important to note that acetazolamide is an adjunct to — not a replacement for — the most important treatment step, which is halting ascent and resting at the current altitude or descending if symptoms are moderate to severe.
A Comparison of Altitude Sickness Medications
The table below provides a comparative overview of the main medications used for altitude sickness prevention and treatment, to help inform your conversation with our pharmacists at Jasins Pharmacy in Hounslow.
| Medication | Primary Use | Prescription Required? | Key Notes |
|---|---|---|---|
| Acetazolamide (Diamox) | Prevention and treatment of acute mountain sickness (AMS) | Yes | Gold standard treatment with the strongest clinical evidence. |
| Dexamethasone | Treatment of severe AMS and HACE | Yes | Reserved for emergency use; not routinely used to prevent altitude sickness. |
| Nifedipine | Prevention and treatment of HAPE | Yes | Typically used for people at high risk of high-altitude pulmonary oedema. |
| Ibuprofen | Relief of AMS headache | No | Helps relieve symptoms but does not prevent altitude sickness. |
| Paracetamol | Relief of AMS headache | No | Provides symptom relief only and does not prevent AMS. |
| Coca leaf / herbal remedies | Traditional remedies | No | No robust clinical evidence supports their effectiveness. |
This table is for informational purposes only. Always consult a pharmacist or doctor before starting any altitude sickness medication, as suitability varies between individuals.
Side Effects of Acetazolamide
Acetazolamide is generally well tolerated, but it does have a characteristic set of side effects that virtually all users will notice to some degree. Being aware of these in advance will help you distinguish between medication side effects and altitude sickness symptoms — an important distinction to make when you are at altitude.
Common Side Effects
The most universal side effect of acetazolamide is tingling or numbness in the fingers, toes, and face (paraesthesia), which occurs in the majority of people who take the medication. This is entirely harmless and, once you know to expect it, is easily distinguishable from any concerning neurological symptom. Increased urination is another very common effect, which occurs because the medication was originally developed as a diuretic. This makes staying well hydrated particularly important when taking acetazolamide at altitude.
Other common side effects include a metallic or altered taste to carbonated drinks, mild fatigue in some individuals, and occasional nausea. Acetazolamide can also increase sensitivity to sunlight, making high-factor sun protection even more important at altitude where UV exposure is already significantly elevated.
Who Should Not Take Acetazolamide
Acetazolamide is a sulfonamide-derived drug. A history of sulfonamide allergy should be discussed with a prescriber before taking acetazolamide, as cross-reactivity is possible but not reliably established — individual assessment is required. It should also be used with caution or avoided in people with kidney or liver disease, those taking lithium, and those with certain electrolyte abnormalities. It is not recommended during the first trimester of pregnancy. This is precisely why a consultation before starting acetazolamide is essential — our pharmacists at Jasins Pharmacy in Hounslow will take a thorough medical history before issuing a prescription.
Medication is Not a Substitute for Acclimatisation
One of the most important things to understand about altitude sickness tablets is that they are a supplement to good acclimatisation practice, not a replacement for it. Acetazolamide accelerates the body’s natural adaptation to altitude — it does not eliminate the need to ascend gradually, take rest days, and respond appropriately to symptoms when they arise.
The fundamental principle of safe high-altitude travel is encapsulated in the phrase “climb high, sleep low” — spending time at progressively higher altitudes during the day before returning to a lower altitude to sleep. Most well-designed trekking itineraries to destinations such as Everest Base Camp or Kilimanjaro build in acclimatisation days for exactly this reason. As a general guide, above 3,000 metres trekkers should aim not to increase their sleeping altitude by more than 300 to 500 metres per day, and should take a rest day for every 1,000 metres of altitude gained above 3,000 metres. (A rest day for every 1,000 metres gained is a widely recommended practical rule that helps prevent AMS from developing as you ascend.)
Adequate hydration is another key component of acclimatisation — drinking two to three litres of water per day at altitude helps the body adjust and reduces the severity of AMS symptoms. Avoiding alcohol and sedatives during the acclimatisation period is also strongly advised, as both can suppress respiration and worsen oxygen desaturation during sleep.
To discuss the right altitude sickness medication for your trek and get a private prescription sorted before you travel, call us to book a consultation with our team at Jasins Pharmacy in Hounslow today.
Other Medications for Altitude Emergencies
For serious altitude illness beyond mild to moderate AMS, two further prescription medications are sometimes carried by experienced high-altitude trekkers and expedition teams as emergency treatments.
Dexamethasone
Dexamethasone is a powerful corticosteroid that can rapidly reduce cerebral oedema and is used as an emergency treatment for HACE and severe AMS. It works quickly and dramatically and can provide a lifesaving window to allow evacuation or descent to a lower altitude. However, it is not recommended for routine prophylaxis of AMS because it suppresses symptoms without addressing the underlying physiology — meaning the condition can rebound severely if ascent continues or the drug is stopped. It is best thought of as an emergency bridge to descent, not a preventative medication.
Nifedipine
Nifedipine is a calcium channel blocker used for the prevention and treatment of HAPE in individuals who are known to be susceptible. It reduces pulmonary artery pressure and can be highly effective in preventing HAPE recurrence in those with a previous episode. Like dexamethasone, nifedipine is a prescription medication and is not appropriate for general altitude sickness prevention — but for those with a documented history of HAPE, carrying it as part of an emergency kit on high-altitude expeditions may be recommended by a specialist.
If you have a history of altitude illness or are planning a particularly ambitious ascent, contact us to speak to our team at Jasins Pharmacy in Hounslow about your individual risk profile and medication options.
Frequently Asked Questions
Here are the altitude sickness tablet questions our travel health team at Jasins Pharmacy in Hounslow is asked most often by trekkers and expedition travellers.
Can I get acetazolamide over the counter without a prescription?
No — acetazolamide (Diamox) is a prescription-only medication in the UK and cannot be purchased over the counter. At Jasins Pharmacy in Hounslow, our travel clinic team can conduct a private consultation and, if appropriate, issue a private prescription for acetazolamide without the need for a GP referral.
When should I start taking altitude sickness tablets before my trek?
The standard recommendation is to begin taking acetazolamide one to two days before ascending to altitude and to continue for at least two days after reaching your highest point or until you begin descending. Starting the medication at least one to two days before ascent allows you to assess how your body responds to the drug before you are at altitude.
Is it safe to take altitude sickness tablets if I am already on other medication?
Acetazolamide can interact with a number of other medications, including lithium, metformin, and certain anticonvulsants, so it is essential to disclose your full medication list during your consultation. Our pharmacists at Jasins Pharmacy in Hounslow will review any potential interactions before issuing a prescription.
Will altitude sickness tablets make me feel unwell?
The most common side effect — tingling in the fingers, toes, and face — is harmless but noticeable, and increased urination is also very common. These effects are manageable and for most people are a worthwhile trade-off for the significant reduction in altitude sickness risk that acetazolamide provides.
Can altitude sickness tablets be taken during pregnancy?
Acetazolamide is not recommended during the first trimester of pregnancy due to theoretical teratogenic risks, and travel to high altitude during pregnancy carries its own significant risks to both mother and baby. If you are pregnant or planning to become pregnant and are considering high-altitude travel, you should seek specialist medical advice well in advance of your trip.
What should I do if I develop altitude sickness despite taking the tablets?
If you develop symptoms of AMS despite taking acetazolamide, the most important step is to stop ascending and rest at your current altitude until symptoms resolve. If symptoms are moderate to severe, or if there is any sign of HACE or HAPE, descent is the only appropriate action — do not wait for symptoms to worsen, and seek emergency medical assistance as quickly as possible.
Don’t Let Altitude Sickness Stop Your Adventure
Altitude sickness is a genuine risk for anyone travelling to high-altitude destinations, but with the right preparation it is largely manageable — and in many cases preventable. Acetazolamide remains the most effective and evidence-based altitude sickness tablet available, and for travellers undertaking rapid ascents or visiting destinations above 3,000 metres, it is a medication worth serious consideration. Combined with a sensible acclimatisation schedule, adequate hydration, and a clear understanding of when to stop ascending or descend, it can make the difference between a successful expedition and a trip cut short by illness.
At Jasins Pharmacy in Hounslow, our travel health team has helped countless trekkers and adventurers prepare for high-altitude journeys, from Everest Base Camp to Kilimanjaro and beyond. We offer private altitude sickness consultations and prescriptions, with same-week appointments available and no need for a GP referral. Whether you are a first-time trekker or a seasoned mountaineer, our pharmacists are here to help you reach your summit safely.
Book your altitude sickness consultation at Jasins Pharmacy in Hounslow today and set off on your adventure with the right preparation behind you.
Disclaimer: The information provided in this blog post is intended for general informational purposes only and should not be treated as medical advice. Always consult a qualified medical professional, pharmacist, or GP before starting any medication, including acetazolamide, particularly if you have underlying health conditions, are pregnant, or are taking other regular medications. If you experience severe symptoms of altitude sickness including loss of coordination, confusion, severe breathlessness, or loss of consciousness, descend immediately and seek emergency medical assistance.
