Kilimanjaro Altitude Sickness: AMS, HACE, HAPE, Diamox, and the Summit Night Decision

Altitude sickness ends more Kilimanjaro climbs than any other cause. Not cold, not injury, not fitness failure — altitude. And the frustrating truth is that it cannot be completely prevented. What you can do is understand how it works, choose a route that gives your body time to adapt, know the warning signs, and make the right call when it matters.

I made the wrong call once, and then the right one. Here is what I learned.

What altitude sickness is — and why Kilimanjaro is particularly challenging

Altitude sickness occurs because at altitude the atmospheric pressure is lower, which means each breath contains fewer oxygen molecules. Your body responds immediately by increasing breathing rate and heart rate. Over 24–72 hours, it begins producing more red blood cells to carry what oxygen is available — the process of acclimatization. If you ascend faster than your body can complete that process, altitude sickness develops.

The critical point that most people misunderstand: altitude sickness is not related to fitness. Very fit, experienced mountaineers get altitude sickness. Olympic athletes get altitude sickness. What determines your susceptibility is how fast you ascend, your individual physiology, and partly genetics. There is no way to train your way out of it.

Kilimanjaro is particularly challenging because of its profile. Uhuru Peak stands at 5,895 m — the highest point in Africa — and most commercial routes reach the summit in 5–8 days from the trailhead. That is fast for that altitude. For comparison, Kilimanjaro’s summit is roughly the same elevation as Everest Base Camp, which expeditions typically take 10–14 days just to reach, with multiple acclimatization days built in.

Altitude sickness on Kilimanjaro can begin as low as 2,500 m. Susceptibility varies significantly by individual, but the mountain will test every climber above 4,000 m. On a 5-day itinerary, AMS rates reach 75–77%. On a 6-day itinerary, that drops to 53% — still more than half of all climbers. Route length is the most powerful tool you have.

The three levels — AMS, HACE, HAPE

Understanding all three levels matters because each requires a different response, and the difference between “manageable” and “medical emergency” can develop in hours.

AMS (Acute Mountain Sickness) is the common entry point. Symptoms: headache that doesn’t resolve with rest and paracetamol, plus at least one of nausea or vomiting, fatigue or weakness, dizziness, or poor sleep. AMS typically begins above 2,500 m and worsens with further ascent. The correct response to AMS is to stop ascending, rest at the same altitude, hydrate, and take ibuprofen or paracetamol for the headache. If symptoms improve over 12–24 hours, cautious ascent may be possible. If they don’t improve — or worsen — descend immediately.

HACE (High Altitude Cerebral Edema) is AMS progressing to brain swelling. It is a medical emergency. Symptoms: severe headache that does not respond to analgesics, ataxia (the inability to walk in a straight line — the standard test is walking heel-to-toe along a straight line), confusion, disorientation, hallucinations, and extreme fatigue to the point of not being able to stand. A climber with HACE who does not descend immediately can die. The only treatment is descent. Supplemental oxygen buys time while getting off the mountain but does not cure HACE.

HAPE (High Altitude Pulmonary Edema) is fluid accumulating in the lungs. Also a medical emergency, and notably, HAPE is the main cause of death on Kilimanjaro. It can occur without preceding AMS — meaning a climber who feels fine may develop HAPE without warning signs. Symptoms progress: first decreased exercise tolerance (you are slow, unusually breathless on mild effort), then a dry cough, then shortness of breath at rest, then frothy or pink-tinged sputum and a crackling sound in the lungs. A climber at rest who is breathless is a HAPE emergency. Descent immediately.

For both HACE and HAPE: the longer you wait, the worse it gets. Time at altitude is the enemy.

The critical altitude thresholds on Kilimanjaro

The numbers on Kilimanjaro help frame the physiological challenge:

Uhuru Peak: 5,895 m. At this altitude, normal blood oxygen saturation (SpO2) — which runs 95–100% at sea level — drops to roughly 70–75%. Your blood is carrying only about two-thirds of its normal oxygen load. This is the extreme of what the body can manage without supplemental oxygen over short periods.

Barafu Camp: 4,673 m. This is where most summit night pushes begin. Climbers spend one or two nights here before the summit attempt. The altitude already produces significant physiological stress — sleep at Barafu is typically fragmented due to periodic breathing (the body’s altitude-induced sleep breathing pattern), even in people who feel well.

The summit night gain: 1,222 m in a single push. From Barafu (4,673 m) to Uhuru Peak (5,895 m) is 1,222 m of vertical gain, covered in approximately 6–8 hours starting at midnight. This is the most aggressive altitude gain of the entire climb — and it comes at the end of 4–6 days on the mountain when sleep deprivation and caloric deficit are at their maximum.

Most turnarounds on Kilimanjaro happen on this night.

There is also a significant logistical constraint: helicopters cannot fly above 5,000 m on Kilimanjaro. If a climber develops a serious emergency above that altitude, they must descend under their own power or be carried to an evacuation point below 5,000 m before helicopter rescue becomes possible. Kilimanjaro MedAir can initiate helicopter evacuation once a climber reaches a lower camp such as Barafu, but above the 5,000 m mark, the team is on its own.

Routes and acclimatization

The route you choose is the most powerful altitude sickness variable within your control. Every additional day on the mountain improves acclimatization and measurably improves your summit success rate.

The data is stark:

Lemosho (8-day) has the best acclimatization profile of any commercially accessible route. It approaches from the west, gaining altitude gradually over the first three days through lower camp elevations. Day 3 on the standard 8-day itinerary includes the “climb high, sleep low” benefit: ascending to Lava Tower (~4,600 m) before descending to Barranco Camp (~3,950 m) for the night. This single day — gaining high altitude and sleeping low — is worth more physiologically than an extra day of flat walking. The 8-day Lemosho begins at 2,100 m (Lemosho Gate), giving the body a full extra day at low altitude compared to the 7-day version, which begins on the Shira Plateau at 3,414 m. That difference matters.

Machame (7-day) shares the Lava Tower acclimatization benefit and is the most popular route on the mountain. A 7-day Machame is a stronger choice than a 6-day one for the same reason: the extra day at altitude before the summit push.

Marangu (5–6 days) is the worst acclimatization profile available. It is the only route with hut accommodation instead of tents, which draws first-time climbers — but its short profile, in-and-out track (no “climb high, sleep low” variation), and minimal acclimatization time make it physiologically the hardest route despite its “beginner” reputation. The 5-day Marangu carries some of the lowest summit success rates on the mountain.

The core principle: the cheapest route is the one that produces the most turnarounds. Saving USD 100–150 in park fees by booking a 5-day rather than 6-day itinerary, or a 6-day rather than 7-day, is a false economy if it means turning around at 5,400 m.

Diamox — what it does and how to use it

Diamox (acetazolamide) is the most studied medication for altitude sickness prevention and is widely used on Kilimanjaro. Multiple independent guidelines — the CDC Yellow Book, the Wilderness Medical Society, and published peer-reviewed studies — converge on the same dose: 125 mg twice daily.

How it works: Diamox is a carbonic anhydrase inhibitor. It mildly acidifies the blood, which stimulates the respiratory center in the brain to breathe faster and more deeply. This accelerates the acclimatization process by getting more oxygen into the system, and it also reduces the periodic breathing pattern that fragments sleep at altitude.

How to take it:

• Start 24–48 hours before ascending above 2,500 m
• Continue at 125 mg twice daily during the ascent and for 2 days at highest altitude
• Drink more water than usual — Diamox increases urination

Common side effects (expected, not dangerous):

• Tingling in fingers, toes, and lips — very common, not a sign of a problem
• Increased urination — stay well hydrated
• Carbonated drinks taste flat — a minor inconvenience, but reported by most people taking it

Who should not take Diamox:

• People with sulfonamide allergy — Diamox is a sulfa drug and cross-reactivity is possible
• People with kidney disease
• Anyone on certain diuretics or aspirin at high doses

Consult a doctor before taking Diamox and test for side effects at home before the climb, since some Diamox side effects can overlap with altitude sickness symptoms (tingling, nausea).

Critical caveat: Diamox is not a substitute for acclimatization. It helps, meaningfully. But a 5-day Marangu ascent on Diamox is still a 5-day Marangu ascent. The medication reduces symptoms; it does not replace the time your body needs.

Summit night — the highest-risk phase

Summit night is when everything converges. Understanding what is happening to you physiologically during this 6–8 hour push helps you recognize the warning signs before they become emergencies.

The departure from Barafu Camp is typically around midnight. You are already sleep-deprived after days at altitude — sleep quality at high camp is poor even for well-acclimatized climbers due to periodic breathing. You have been eating less than usual because appetite suppresses at altitude. You are starting in complete darkness at temperatures of -10°C to -20°C, with wind chill potentially much colder on exposed ridges.

And then you climb 1,222 m.

As you gain altitude above Barafu, you are moving into territory your body has never been at before. The acclimatization you built over the previous days was at 4,600 m; you are now pushing to 5,895 m. The physiological response is not gradual — your SpO2 continues to drop, your breathing becomes labored with each step, and the combination of cold, exertion, and oxygen debt produces a cumulative deficit that compounds by the hour.

Warning signs on summit night that mean turn around now:

• Ataxia: You cannot walk in a straight line. This is the clearest sign of HACE beginning. If your guide puts you to the heel-toe test and you fail it, descent is immediate.
• Unresponsive headache: A headache that has not improved with ibuprofen and paracetamol taken 45–60 minutes earlier, especially if it is severe.
• Confusion or disorientation: If you are not thinking clearly, if you do not know where you are or what day it is, if your behavior seems unusual to your guide.
• Vomiting that prevents hydration: Not just nausea — vomiting so frequent that you cannot keep water down. Dehydration accelerates every other symptom.
• Shortness of breath at rest: If you stop moving and are still gasping for breath, that is a HAPE warning.

The darkness makes these signs harder to recognize. Guides trained on Kilimanjaro carry pulse oximeters and check oxygen saturation twice daily during the climb. A SpO2 reading significantly below the expected range for that altitude, combined with symptoms, is grounds for descent. Listen to your guide.

The golden rule, oxygen, and evacuation

Descent is the only treatment. This is not a metaphor or a simplification — it is physiology. For both HACE and HAPE, the only thing that cures the underlying condition is reducing altitude. Supplemental oxygen provides temporary improvement and can allow a climber to walk down rather than be carried, but it does not cure either condition. A climber on supplemental oxygen who remains at altitude will continue to deteriorate.

The improvement from descent is often dramatic and fast. A recommendation in the literature for severe AMS is to descend to around 600 m (2,000 ft) below the point where symptoms began. In practice on Kilimanjaro, even descending 300–500 m can produce marked improvement within 1–2 hours. Climbers who cannot stand at altitude often walk off the mountain under their own power once they reach lower camp.

Most reputable Kilimanjaro operators carry supplemental oxygen and a Gamow bag (a portable hyperbaric chamber that simulates descent by inflating around a patient) on every climb. These are emergency tools — they buy time while getting off the mountain, not alternatives to descent.

Helicopter evacuation is available through Kilimanjaro MedAir, but with a significant constraint: helicopters cannot fly above 5,000 m. A climber at Uhuru Peak (5,895 m) or on the summit night approach cannot be reached by helicopter until they have been brought down to approximately Barafu Camp (4,673 m). This means an emergency on summit night requires the team to manage descent at altitude, in the dark, often in cold, before rescue by air is possible. AMREF Flying Doctors coverage or equivalent medevac insurance is not optional on Kilimanjaro — it is what funds a helicopter at Barafu if you need it.

My 5,400 m turnaround

On my first Kilimanjaro attempt, I started summit night feeling reasonable. I had a headache at Barafu that afternoon — not severe, just present. I took ibuprofen, drank water, ate what I could, and rested. By midnight it had improved enough that I decided to push.

Ninety minutes out of Barafu, at roughly 5,400 m, it came back. Not dramatically — just a steady pressure that was returning to where it had been, with effort in every step that felt heavier than effort at 4,600 m had. I told my guide. He looked at me carefully, shone his headlamp on my face, asked me to step in a straight line.

I managed the line. But I realized as I tried that I was leaning against him without having decided to. I had drifted without noticing it.

We went down.

Back at Barafu, two hours later, the headache was gone. By breakfast at lower camp, I felt entirely normal — which is both the miracle of descent and a lesson in how rapidly the body recovers when you give it the altitude it can handle.

I have thought about that decision many times. The guide made it as much as I did — he saw something I couldn’t see from inside my own head. That is why the guide matters. A good guide on Kilimanjaro is not a walking companion. He is the person who makes the call you may not be clear-headed enough to make yourself.

When in doubt, go down. The mountain is still there.

For the full route comparison — Lemosho vs Machame vs Marangu vs Northern Circuit, day-by-day camps, and choosing by acclimatization profile — see the Kilimanjaro routes guide. For the physical preparation — the 16-week training plan, what fitness helps and what doesn’t, and how to prepare for the altitude specifically — see the Kilimanjaro training guide. For the full summit kit — the 3-layer system, what to wear at midnight at -20°C, and what not to bring — the Kilimanjaro packing guide covers every item. For the broader health picture before any Tanzania trip — vaccinations, malaria prophylaxis, medical insurance — see the Tanzania health guide.