How does altitude sickness work — and when do you descend?

TL;DR. On any high Nepal trek the real danger isn’t the walking — it’s the altitude. Everest Base Camp tops out at 5,320 m and the Annapurna Circuit’s Thorong La at 5,416 m, and the risk zone is everything above ~3,000 m. The fix is not fitness or toughness; it’s going up slowly: above 3,000 m don’t raise your sleeping altitude by more than ~500 m a night, take your acclimatisation days, climb high and sleep low, and never ascend with symptoms. The one rule that saves lives: if it’s getting worse, descend.

⚠️ This is educational planning information, not medical advice. Altitude illness can be fatal. Consult a doctor before your trek — especially about medication — and seek qualified help on the ground if you’re unwell. When in doubt, go down.

Quick answers

Why altitude is the risk on this trek

The Annapurna Circuit is a walk, not a climb — no ropes, no technical ground. What makes it serious is height: from Manang (~3,540 m) you spend days above 3,000 m and cross Thorong La at 5,416 m, where the air holds roughly half the oxygen of sea level. Altitude illness doesn’t care how fit or experienced you are; it’s about how fast you went up. That’s the whole reason the standard 14-day itinerary slows down in its second half rather than racing to the pass.

The three forms of altitude illness

AMS is your early warning system — respect it and it rarely becomes anything worse. Ignore it and push higher, and it can progress to HACE or HAPE, which are emergencies.

The golden rules of acclimatisation

These are the consensus rules (per the Wilderness Medical Society) the Circuit’s itinerary is built around:

1. Ascend slowly. Above ~3,000 m, don’t increase your sleeping altitude by more than ~500 m per night.
2. Build in rest days. Take an acclimatisation day every 3–4 days (or after ~1,000 m of cumulative sleeping-altitude gain) — this is exactly what the Manang day is for. Don’t skip it to save time.
3. Climb high, sleep low. Day-hike higher than where you’ll sleep, then come back down to sleep — the single best active acclimatisation tactic.
4. Never go higher with symptoms. If you have AMS, stay put until it fully resolves. No views are worth pushing through it.
5. If it’s getting worse, descend. Going down even 300–1,000 m is the most effective treatment there is.
6. Eat, drink, and don’t mask it. Stay hydrated and fed; avoid alcohol and sleeping pills, which can blunt the warning signs.

How the Circuit’s itinerary protects you

The classic 14-day pacing isn’t padding — it’s an acclimatisation plan. Walking anti-clockwise (Besisahar first) means the climb to Thorong La is gradual over many days, and the Manang rest day lands right before the highest sleeping altitudes. The shape is a long, patient climb, not a sprint. The fast 8–10 day versions are riskier precisely because they compress this — if you trek one, you’re trading away acclimatisation, so watch yourself closely.

Medication: a doctor’s call, not ours

Acetazolamide (Diamox) can help prevent AMS and is the one drug that actively aids acclimatisation; dexamethasone is also used in prevention and treatment. But dosing and suitability are medical decisions — we won’t prescribe. Talk to a doctor or a travel clinic before you go about whether to carry them and how to use them, alongside any personal health conditions. Medication is a backup to slow ascent, never a substitute for it.

When to turn around

This is the hardest and most important judgement on the trek. Descend immediately, and get help, if you or anyone in your group has:

• Worsening symptoms despite resting at the same altitude
• Confusion, drowsiness, or loss of coordination (HACE)
• Breathlessness at rest, a wet cough, or chest tightness (HAPE)

Carry travel insurance with helicopter-evacuation cover to at least 5,500 m — on a 5,416 m pass it’s essential, not optional. Turning back is not failure; it’s the experienced call.

Cold makes it harder

High altitude and cold arrive together: the pass morning is both thin-aired and bitterly cold, and being cold, dehydrated, and exhausted makes altitude harder to handle. Staying warm and functional is part of staying safe up high — our cold-weather training guide covers building tolerance and a layering system.

Our honest position

We haven’t crossed Thorong La yet — that’s November 2026. The acclimatisation principles above are sourced from established mountain-medicine guidance and aren’t ours to reinvent; after we walk it, we’ll add first-hand notes on how the pacing actually felt and how we managed the pass day. The rules don’t change with the season — but the lived detail does, and that’s what we’ll add.

Sources & notes

• Altitude-illness types, ascent-rate and acclimatisation rules, descent guidance: Wilderness Medical Society Clinical Practice Guidelines for the Prevention, Diagnosis, and Treatment of Acute Altitude Illness (2024 update), and its summary in American Family Physician.
• Status: educational summary of established mountain-medicine consensus, current to June 2026. Not medical advice — consult a doctor, especially about medication. First-hand pass-day notes to be added after the author’s November 2026 trek.

Educational planning content, not medical advice — altitude illness is serious and can be fatal; consult a qualified doctor before you travel and seek help on the ground if unwell. Some links are affiliate links; we only recommend what we’d use ourselves, at no extra cost to you. © 2026 keitreks · The numbers are measured. The trail is next.

Related: Road to the Annapurna Circuit · Cold-weather training · Permits & the guide rule