Altitude Sickness Guide · Himalayas 2026
The First-Timer's Guide to
Altitude Sickness — And How We Prevent It
What it actually is, who actually gets it, and the exact approach we use to keep every client safe above 3,500 metres.
⚕ Medical disclaimer: This guide is written from 15 years of high-altitude operational experience. It is not a substitute for medical advice. Before any high-altitude trek, consult a doctor — particularly regarding Diamox and any pre-existing conditions.
The Honest Answer
Altitude sickness is real, it is manageable, and with the right itinerary and operator it should not stop you from trekking in the Himalayas. The vast majority of cases are mild and resolve with rest and proper acclimatisation. Serious cases are rare and almost always the result of ascent that is too fast, not an inherent physical weakness. In 15 years of operations, we have never lost a trekker to altitude illness — because we take it seriously before it becomes a problem.
Altitude sickness is the most common reason first-timers hesitate before booking a Himalayan trek. It is also the most misunderstood. Fear of it sends some people to high altitude without proper preparation. Exaggerated fear of it stops others from going at all. Neither response is right — and both are preventable with accurate information.
What Is Altitude Sickness? The Simple Physiology
At sea level, every breath you take delivers oxygen to your lungs under normal atmospheric pressure. As you ascend, atmospheric pressure drops — and with it, the partial pressure of oxygen in each breath. At 3,500m, you are breathing air with roughly 65% of the oxygen available at sea level. At 5,000m, that figure drops to around 53%.
Your body can adapt to this — but it needs time. The adaptation process, called acclimatisation, involves your body producing more red blood cells, increasing your breathing rate, and adjusting blood chemistry to carry oxygen more efficiently. This process takes days, not hours.
When you ascend faster than your body can adapt — the most common cause of altitude illness — your brain and lungs begin to experience oxygen deficiency. The symptoms that result are collectively called Acute Mountain Sickness (AMS) at their mildest, and can escalate to life-threatening conditions if the underlying cause (being too high, too fast) is not addressed.
"The mountain doesn't care how fit you are. It cares how fast you went up."
— Standard wisdom among high-altitude guides, repeated on every trek we run
The critical insight — and the one most commonly ignored — is that altitude sickness has nothing to do with fitness. Elite athletes get it. Sedentary first-timers sometimes don't. The single biggest predictor is rate of ascent. Everything else is secondary.
The Three Types: AMS, HACE, and HAPE
Altitude illness exists on a spectrum from uncomfortable to life-threatening. Understanding the three distinct conditions helps you respond appropriately rather than either ignoring early symptoms or panicking unnecessarily.
- Headache (primary symptom)
- Fatigue and weakness
- Dizziness or lightheadedness
- Nausea, loss of appetite
- Poor sleep at altitude
- Onset typically 6–12 hrs after ascent
- Severe, worsening headache
- Confusion, disorientation
- Loss of coordination (ataxia)
- Extreme fatigue
- Altered consciousness
- Fluid on the brain — descent required immediately
- Breathlessness at rest
- Persistent dry cough
- Pink or frothy sputum
- Blue-tinged lips (cyanosis)
- Gurgling sounds in chest
- Fluid in the lungs — descent + emergency care required
AMS is common and manageable. HACE and HAPE are rare but medical emergencies that require immediate descent — they cannot be waited out, and they will not resolve at altitude without intervention. The good news: both HACE and HAPE are almost always preceded by AMS symptoms that, if taken seriously and acted upon, prevent escalation entirely.
The Golden Rule of Altitude — Non-Negotiable
Never ascend with AMS symptoms. If you have a headache, nausea, or significant fatigue at altitude, stay at your current elevation until symptoms resolve. If symptoms worsen despite staying put, descend. Altitude sickness does not "push through" — it escalates. This is the rule our guides enforce without exception, regardless of weather windows, group pressure, or schedule constraints.
Symptoms: From Mild Warning to Medical Emergency
The Lake Louise Score is the standard clinical tool for assessing AMS severity. Our guides are trained to assess it informally but accurately. Here is how to read the progression yourself.
Who Gets Altitude Sickness — And Who Doesn't
This is the most important thing we tell every client before departure, because it is the thing most commonly misunderstood.
What Does NOT Predict Altitude Sickness
- Physical fitness: Cardiovascular fitness does not protect you. Marathon runners get AMS. Untrained hikers sometimes don't. Fitness affects how you feel on the trail — it does not affect your body's oxygen-carrying capacity at altitude.
- Age: Younger trekkers are not more protected. In fact, some research suggests younger people are more susceptible, possibly because they push harder and ascend faster.
- Previous experience at altitude: Having been fine at altitude before does not guarantee you will be fine again. Individual susceptibility varies trip to trip, depending on how quickly you ascended and your physiology at that time.
- Gender: No significant difference in susceptibility between men and women.
What DOES Help Reduce Susceptibility
- Rate of ascent: The most controllable factor. Slow ascent profiles dramatically reduce incidence of AMS. This is why our itineraries include mandatory acclimatisation days.
- Prior altitude exposure within 1–2 months: Recent time at altitude (e.g., spending a week at 3,000m before attempting 5,000m) does provide some protection through physiological priming.
- Hydration: Dehydration compounds altitude effects significantly. Proper fluid intake is one of the few genuinely controllable variables.
- Avoiding alcohol and sedatives: Both suppress breathing rate during sleep, reducing overnight oxygen intake at precisely the time your body needs it most.
- Knowing your own history: If you have had significant AMS before, tell us. Not so we discourage you — so we can design a more conservative itinerary.
The 5 Things That Actually Prevent Altitude Sickness
There is a lot of questionable advice online about altitude sickness prevention. Here is what the evidence — and our operational experience — actually supports.
Ascend slowly — "climb high, sleep low"
Above 3,000m, the standard guideline is to increase your sleeping altitude by no more than 300–500m per day, with a rest day every 3–4 days of ascent. The "climb high, sleep low" principle — ascending to a higher elevation during the day, then returning to sleep at a lower camp — is used on most serious high-altitude expeditions for exactly this reason. Our Kashmir Great Lakes and Everest Base Camp itineraries are built around this principle explicitly.
Stay hydrated — genuinely and consistently
At altitude, your body loses water faster through increased respiration and reduced thirst sensation. Most trekkers underdrink without realising it. The target is 3–4 litres of water per day during active trekking. Urine colour is the honest test: pale yellow means adequately hydrated; dark yellow means you are behind. Our cooks provide hot water at every camp — use it.
Avoid alcohol, sleeping pills, and heavy sedatives
All three suppress your respiratory drive — the unconscious reflex that keeps you breathing appropriately during sleep. At altitude, where your sleeping oxygen saturation is already lower than at sea level, this suppression can be significant. Many trekkers who report "terrible nights at altitude" are experiencing the combined effect of altitude and the glass of whisky they had at dinner. Abstain above 3,000m, especially in the first 48–72 hours at any new elevation.
Listen to your body — and tell your guide the truth
This sounds obvious. It is not. There is enormous psychological pressure on group treks to not be "the one who slows everyone down." Trekkers regularly underreport symptoms to avoid this perception. In 15 years, every serious altitude situation we have managed involved a client who had symptoms earlier than they reported them. Our guides ask directly, every morning. Answer honestly — it is what we are there for.
Eat, even when you don't want to
Altitude suppresses appetite. The body needs significantly more energy at altitude to maintain warmth and fuel altitude-specific physiological adaptations. Trekkers who stop eating properly at altitude deteriorate faster and experience AMS symptoms more severely. Force yourself to eat carbohydrate-rich meals, particularly in the first few days at a new elevation. Our cooks prepare food specifically calibrated for this — use it.
Diamox: The Honest Guide
Acetazolamide — sold as Diamox — is the most commonly discussed pharmaceutical option for altitude sickness prevention and treatment. It is also the most commonly misunderstood. Here is what it actually does, and when we recommend it.
How Diamox Works
Acetazolamide is a carbonic anhydrase inhibitor. It works by stimulating faster, deeper breathing — effectively accelerating the body's natural acclimatisation response. It does not oxygenate your blood directly; it makes your body work harder to oxygenate itself. The result is that trekkers on Diamox typically acclimatise faster and sleep better at altitude.
| Question | Answer |
|---|---|
| Is it safe? | Yes, for most people. It is a well-studied drug with a long safety record. Contraindicated in sulfa drug allergy and in people with kidney problems. Consult your doctor before use. |
| Who should consider it? | Trekkers with a history of significant AMS, those on compressed itineraries with limited acclimatisation days, or anyone with known susceptibility who wants an additional safety layer. |
| Who doesn't need it? | Most trekkers following a conservative acclimatisation itinerary do not require Diamox as a preventive. It is not a substitute for proper ascent pacing. |
| What are the side effects? | Increased urination (significant — plan toilet access accordingly). Tingling in fingers and toes. Carbonated drinks taste flat. Rare: nausea, fatigue at onset. |
| Typical preventive dose | 125mg twice daily, starting 1–2 days before ascending above 3,000m. Your doctor may prescribe 250mg. Follow medical advice over general guidelines. |
| Can it mask symptoms? | Partially — this is the main concern with prophylactic use. It can reduce headache severity, potentially masking early AMS warning signals. This is why guides monitor all clients regardless of whether they are on Diamox. |
| Where to get it | Prescription required in most countries. Get a prescription from your GP before travel. Available over the counter in India and Nepal but we recommend getting it before departure. |
Our position: Diamox is a useful tool in specific circumstances. It is not a pass that lets you ignore acclimatisation principles or push your ascent faster than your body can handle. We encourage clients who are considering it to discuss it with their doctor and with us before the trek.
Questions About Altitude Safety on Your Trek?
Talk to our guides before you book. We'll tell you exactly what to expect on your specific route, what acclimatisation profile we use, and whether Diamox might be relevant for your situation.
How We Build Acclimatisation Into Every Trek
The difference between a safe and unsafe high-altitude trek is not luck or fitness — it is itinerary design. Here is exactly how we approach acclimatisation on our two main high-altitude routes.
Kashmir Great Lakes Trek — Acclimatisation Profile
The Kashmir Great Lakes circuit reaches a maximum elevation of approximately 4,300m. Our 8-day itinerary is designed around the following ascent profile:
| Day | Camp / Location | Sleeping Elevation | Notes |
|---|---|---|---|
| Day 1 | Naranag trailhead → Shokhar | ~3,100m | Gradual first day. Short hours. |
| Day 2 | Shokhar → Vishansar Lake | ~3,596m | First alpine camp. Guide monitors all clients. |
| Day 3 | Acclimatisation day at Vishansar | ~3,596m (rest) | Mandatory rest. Optional short walk to Krishansar. |
| Day 4 | Vishansar → Gadsar Pass → Gadsar Lake | ~3,600m | Highest pass day (~4,300m). Sleeping altitude remains moderate. |
| Day 5 | Gadsar → Satsar Lakes | ~3,700m | Gradual. Multiple small lakes en route. |
| Day 6 | Satsar → Gangabal Twin Lakes | ~3,576m | Slight descent for sleeping. Good acclimatisation profile. |
| Day 7 | Gangabal → Naranag descent | ~2,250m | Full descent. Altitude no longer a factor. |
Everest Base Camp Trek — Key Acclimatisation Days
The EBC Trek reaches 5,364m at Base Camp and 5,644m at Kala Patthar — requiring significantly more acclimatisation time than Kashmir routes. Our 14-day itinerary includes two dedicated acclimatisation days that are non-negotiable:
EBC Mandatory Acclimatisation Days
- Namche Bazaar (3,440m) — Day 3–4: Two nights in Namche with an acclimatisation walk to the Everest View Hotel at 3,880m. This is the most important early acclimatisation stop on the entire route. Trekkers who skip it face dramatically higher AMS incidence above Tengboche.
- Dingboche (4,410m) — Day 8–9: Second mandatory acclimatisation stop. Acclimatisation walk to Nangkartshang Peak (5,083m) — the classic "climb high, sleep low" day that prepares the body for the 5,000m+ elevations above.
- These days cannot be compressed or removed regardless of time pressure. We will not shorten EBC itineraries below 12 days — and we recommend 14.
What Happens If Someone Gets Sick on Our Treks
Despite every precaution, altitude sickness happens. Here is our exact protocol when it does — because transparency about this is part of what makes it safe to trek with us.
Our Altitude Emergency Protocol
- Daily morning health checks: Every guide assesses every client each morning — headache, sleep quality, appetite, and coordination. We use a simple version of the Lake Louise Score. Guides are trained to spot underreporting.
- Oxygen availability: We carry supplemental oxygen on all treks above 4,000m. Portable oxygen canisters are for emergency use and symptom assessment, not routine comfort use.
- Medications carried: Our guides carry ibuprofen, paracetamol, Diamox, dexamethasone (for HACE), and nifedipine (for HAPE) on all high-altitude routes. These are used only when clinically appropriate.
- Descent as treatment: For anything beyond mild AMS, controlled descent is the primary treatment — not medication. We will always descend before the situation becomes an emergency, not after. A descent of even 300–500m typically produces rapid improvement.
- Satellite communication: All guides on remote Kashmir and Nepal routes carry satellite communicators. In the event a client cannot self-descend, we can coordinate helicopter evacuation from any location on our route within hours of a distress call.
- We never split groups carelessly: If one client needs to descend, a guide descends with them. The remainder of the group continues only if the lead guide assesses it is appropriate and a second guide is available.
"On day five I had a headache that wouldn't shift and felt off all morning. Our guide spotted it before I mentioned it. We stayed an extra night at that camp, I rested and drank fluids, and the next morning I felt completely normal. We completed the route. I realise now that without that call, I might have pushed into something serious."
— Rachel, EBC Trek, October 2024
Frequently Asked Questions
Ready to Trek — Safely?
Every Summit Routes itinerary is designed with acclimatisation as the priority — not speed, not cost-cutting, not schedule pressure. Our guides are trained to manage altitude illness from early warning to emergency response.
