Annapurna Circuit Altitude Sickness: AMS Prevention & Thorong La Safety

How Real Is the Altitude Risk on the Circuit?

The Annapurna Circuit reaches 5,416 m on the Thorong La, well into the zone where acute mountain sickness (AMS) is a serious risk for anyone, regardless of fitness or age. This is far higher than base-camp treks like Annapurna Base Camp (4,130 m). Two things make the Circuit manageable if you treat it right:

• The route has an excellent acclimatisation profile, you gain altitude gradually over many days up to Manang.
• There is a built-in acclimatisation day at Manang (3,540 m), the single most important day for crossing the pass safely.

That said, AMS is unpredictable and fitness does not protect you. The danger is in rushing the high section or skipping the Manang rest day. The free altitude talk at the Himalayan Rescue Association aid post in Manang is worth attending.

Circuit altitude profile

AMS Symptoms: Mild vs Emergency

Know the difference. Mild AMS is common and manageable; the severe forms (HACE/HAPE) are a genuine risk at the Circuit's altitude and are life-threatening, demanding immediate descent.

The golden rule: if you have symptoms, do not go higher, and never cross the Thorong La while unwell. If symptoms are severe or worsening, descend immediately, even at night. Descent is the cure.

How to Prevent AMS

• Keep the Manang acclimatisation day. This is non-negotiable, never skip it to save time, even on the short 12-day itinerary.
• Ascend gradually above 3,000 m, and climb high and sleep low where you can (for example, a hike toward Ice Lake from Manang).
• Hydrate. Drink 3–4 litres a day; treat your own water to keep drinking freely.
• Walk slowly and steadily, "pole, pole" (slowly, slowly). Let your breathing set the pace, especially on the pass climb.
• Avoid alcohol and sleeping pills high up; they suppress breathing.
• Eat well, even without appetite, carbohydrates help acclimatisation.
• Consider Diamox (acetazolamide) if your doctor advises it. A common preventive dose is 125 mg twice daily, started a day before going high; it speeds acclimatisation. Discuss it with your doctor before the trip, it is not a substitute for sensible ascent.

Crossing the Thorong La Safely

The Thorong La is the defining hazard of the Circuit: very high, very exposed, and notorious for sudden storms. The 2014 blizzard on the pass killed dozens of trekkers and guides caught crossing in a sudden whiteout, a reminder that the pass demands respect.

• Start before dawn so you are over the top and descending before the afternoon wind and cloud build.
• Cross only in clear, settled weather. Never start the pass in a storm or fresh deep snow.
• Be willing to wait or turn back. Build a contingency day at Thorong Phedi; the pass will still be there tomorrow.
• Trek with a licensed guide who reads the weather and recent conditions and will postpone the crossing if needed.
• Dress for sub-zero wind: down jacket, windproof shell, warm gloves and hat, and a headlamp (see packing list).

Insurance & Emergency Action

• Carry travel insurance with helicopter-evacuation cover valid for trekking to at least 5,500 m. This is non-negotiable on a trek that crosses 5,416 m, a heli rescue from the high section costs many thousands of dollars.
• Keep your insurer's emergency number and policy number accessible (and with your guide).
• If someone shows emergency symptoms, descend immediately while arranging help, losing altitude is the priority, not waiting for a helicopter.
• Use the Himalayan Rescue Association aid post at Manang if you have any doubts before going high, and never push over the pass with worsening symptoms.