Kilimanjaro Altitude Sickness Guide

What Altitude Sickness Actually Is

At sea level, barometric pressure is 760mmHg and your blood is roughly 98% saturated with oxygen. At 3,000m — the elevation where most Kilimanjaro routes spend their second and third nights — pressure drops to about 525mmHg and saturation falls to 90%. At Kilimanjaro's summit (5,895m), pressure is 340mmHg and even healthy climbers sit at 75–82% saturation. Every climber at altitude is hypoxic. The question is whether the hypoxia produces symptoms severe enough to stop you.

The body adapts to altitude through four mechanisms: faster breathing (the hypoxic ventilatory response), more red blood cells (erythropoiesis, takes 4–7 days), higher heart rate and cardiac output, and improved oxygen extraction in muscles. The first three start within hours. The last two take days. Kilimanjaro's itineraries compress 4,000–5,000m of vertical gain into 5–9 days, which is faster than the body wants.

Altitude illness exists on a spectrum. Acute Mountain Sickness (AMS) is the mildest, most common form — headache, nausea, fatigue, poor sleep. It is reversible with rest or slow ascent. High Altitude Cerebral Edema (HACE) and High Altitude Pulmonary Edema (HAPE) are the life-threatening forms. Both involve fluid accumulation in the brain or lungs, both progress over hours, and both kill without descent. The entire point of the rest of this guide is to keep you in the AMS category and away from the HACE/HAPE boundary.

How to Recognise Altitude Sickness — By Severity

Symptoms matter in two ways: which ones you have, and how they are changing over time. A stable headache at 4,200m that is not getting worse is very different from the same headache that has gotten worse over 12 hours and is now joined by nausea. Below is the working framework our guides use in the field.

The trend matters more than the absolute severity. A climber with a Lake Louise Score of 4 that stays at 4 over 24 hours is in a different position from a climber whose score has climbed from 2 to 6 over the same window. The first is being monitored; the second is descending. Our guides conduct formal LLS assessments twice daily above 3,500m — once on waking, once at evening briefing — and they track every climber's trend on a small card kept by the lead guide.

For a detailed breakdown of AMS, HACE, and HAPE including the Lake Louise scoring system, see our complete altitude sickness guide. For route-by-route risk profiles and how route choice affects altitude exposure, see our Kilimanjaro routes compared page.

Diamox (Acetazolamide) — What It Does and What It Does Not

Acetazolamide, sold as Diamox, is the most-prescribed altitude medication in the world. Multiple controlled trials show it reduces AMS incidence by 40–50% when taken prophylactically. It works by inhibiting carbonic anhydrase in the kidneys, which increases bicarbonate excretion, mildly acidifies the blood, and triggers faster breathing. The faster breathing accelerates acclimatization.

The standard prophylactic protocol on Kilimanjaro: 125mg twice daily, starting 24 hours before ascent above 3,000m, continuing through summit night, and stopping 48 hours after final descent. Higher doses (250mg twice daily) are used by some operators but cause more side effects without measurably better outcomes.

Diamox is a sulfonamide. If you have a sulfa drug allergy, do not take it. Alternatives exist (dexamethasone, nifedipine) but they are used for treatment of established illness rather than prevention and carry their own risk profiles. Discuss with a travel medicine doctor at least 6 weeks before departure.

For the full Diamox protocol, dosing details, sulfa alternatives, and what to do if your doctor has never prescribed it for altitude, see our Kilimanjaro Diamox guide. The short version: Diamox helps, it is not magic, and it is not a substitute for choosing a longer route.

Pre-Climb Health Checklist — The Month Before You Fly

Most altitude-related problems we see on Kilimanjaro are predictable before the climber ever leaves home. The four weeks before your climb are where you reduce your risk the most.

How a Real Descent Decision Gets Made

We sat down with our head guide — 22 years on Kilimanjaro, over 400 summits, no HACE or HAPE fatalities under his watch — and asked him to walk us through a real descent call. He picked a case from the 2025 climbing season. It involved one of our climbers, a 47-year-old woman, on the Lemosho 8-day route, at 4,600m on day four.

“She woke up at Lava Tower camp with a headache she described as ‘a 7 out of 10.’ She had taken ibuprofen and drunk water. Morning LLS was 5 — headache 2, nausea 1, fatigue 1, dizziness 1. Pulse ox was 81. She had no confusion, no ataxia, no breathlessness at rest. By all the textbook numbers, she was borderline moderate AMS, not HACE.

But she had a history I knew about — she had turned back at 4,800m on a previous climb in Peru, on a different operator, two years before. She did not tell her booking agent. She told me on day two of this climb when we sat down for the medical review. That changed the calculation completely.

The trend is what I watch, not the number. A score of 5 at one camp is acceptable if it was 5 yesterday. A score of 5 that came from 3 yesterday is a climber who is sliding. Her number was 5, but the trajectory was wrong. Pulse ox at 81 was below the threshold. Her prior history meant I had less margin to work with.

I told her we would descend to Barranco Camp for the night — 600m down. She resisted. She said she felt fine, she just wanted a slower morning. I said no. I do not argue with climbers about this. I told her the choice was descend with me now, or I would not continue the climb with her tomorrow even if her numbers improved. She chose to descend.

She slept at 3,900m. Next morning LLS was 2. Pulse ox 88. We re-ascended over two days. She summited on day seven. Her husband, who had pushed on to the high camp, did not summit. He turned back at 5,400m with HAPE symptoms and was evacuated by our guide team to Moshi.

The lesson: she would have summited if I had let her push. Probably. But the trend was wrong and her history was wrong. I would rather bring a climber down for one night than carry one off the mountain in a helicopter. Every time.”

Two things to notice in that account. First, the decision was made on trend, not on a single number. Second, the climber was not asked to vote. Our guides retain final authority on ascent and descent calls. The booking office, the climber's family, and the climber's own confidence in how they feel are all secondary to the guide's read of the situation in real time. This is non-negotiable in our operation. It is why our HACE/HAPE rate is below the published operator average despite climbing with climbers across the full age and fitness spectrum.

The Three Triggers That Mean Descend Now

Forget Lake Louise scores for a moment. There are three symptom patterns that mean descend immediately, regardless of the score, regardless of how close the summit is, regardless of how good you felt yesterday. Our guides are trained to act on these within minutes of recognition — there is no “wait and see” window.

For a full breakdown of how descent works on each route, what happens at each camp if a climber needs to come down, and how evacuation to Moshi or Arusha is handled, see our Kilimanjaro descent guide.

Route Choice Is Altitude Sickness Prevention

The single biggest altitude sickness decision you make is which route and how many days. Published summit success rates across operators correlate strongly with itinerary length: the 6-day Machame averages a strong southern-route acclimatisation profile, the 7-day Machame averages a strong southern-route acclimatisation profile, the 8-day Lemosho averages a strong southern-route acclimatisation profile, and the 9-day Northern Circuit averages a strong southern-route acclimatisation profile. The differences are not marketing. They reflect the time available for your body to adapt.

If you have any history of altitude sickness, are over 50, are climbing with a respiratory condition, or are simply not confident in your fitness for sustained effort, choose the longest route you can afford. The 8-day Lemosho is our most-climbed route for good reason: it gives your body two full days above 4,000m before summit night and includes the Lava Tower acclimatisation day that is the single most physiologically valuable day on the mountain.

See the routes compared page for a side-by-side of every route's itinerary, success rate, altitude profile, and best-fit climber profile.

The Prevention Summary

Eight actions, in priority order. Do all eight and your summit odds rise to roughly 95%. Do three and they fall to roughly 70%. The actions are not equal in weight — the top four matter more than the bottom four — but they compound.

1. Choose the longest route you can afford. 8-day Lemosho or 9-day Northern Circuit beats 7-day Machame beats 6-day Marangu for altitude adaptation.
2. Tell your guide everything. Prior AMS, sulfa allergies, sleep apnea, anxiety at altitude, medications, recent illness. Withheld information removes their ability to protect you.
3. Discuss Diamox with your doctor 6 weeks out. 125mg twice daily starting 24 hours before ascent above 3,000m. Reduces AMS by 40–50% in trials.
4. Climb pole pole. Swahili for slowly slowly. Above 3,000m, do not exceed 500m of sleeping altitude gain per night. Rest days above 4,000m are not optional.
5. Hydrate aggressively. 3–4 litres per day on trek, 5 litres on summit night. Pale yellow urine is your target.
6. Eat carbs at altitude. Your body prefers carbohydrates above 4,000m. 60–70% of your calories should be carb-based.
7. No alcohol until you are back in Moshi. Zero. Altitude dehydrates you. Alcohol dehydrates you faster. The two together are not a small problem.
8. Sleep when you can. Sleep is disrupted at altitude — every climber wakes at 02:00. That is normal. Nap during the day when your body wants to. Do not chase wakefulness.