Altitude sickness is the primary reason for Kilimanjaro summit failure — the mountain’s relatively rapid ascent profile (most routes reach 5,895m in 5–7 days from the 1,800m gate) means trekkers gain altitude faster than is physiologically ideal for proper acclimatisation. Understanding how altitude affects the body, recognising the symptoms of acute mountain sickness (AMS) and its more serious forms (HACE, HAPE), and knowing the evidence-based prevention strategies dramatically increases both summit success rate and safety on the mountain. This guide covers the altitude safety information every Kilimanjaro trekker should understand before departure.
How Altitude Affects the Body
At sea level, each breath of air contains approximately 21% oxygen — but at Kilimanjaro’s summit (5,895m), the atmospheric pressure is approximately 50% of sea level, meaning the same air contains effectively half the oxygen molecules per breath. The body’s initial response to reduced oxygen (hypoxia) is increased breathing rate and increased heart rate — partially compensating for the reduced oxygen per breath by breathing more often and pumping blood faster. Over days at altitude, deeper physiological adaptations occur: increased red blood cell production (taking 1–2 weeks), increased lung capacity, and changes in blood chemistry. The problem on Kilimanjaro is time: most routes spend only 1–2 days above 4,000m before the summit attempt, providing insufficient time for physiological adaptation. The result is a spectrum of altitude illness that affects 75% of Kilimanjaro trekkers to some degree.
AMS: Symptoms and Recognition
Acute Mountain Sickness (AMS) is defined by the Lake Louise Criteria as: headache at altitude plus one or more of the following — loss of appetite/nausea/vomiting, fatigue or weakness, dizziness or light-headedness, difficulty sleeping. AMS symptoms on Kilimanjaro typically appear above 3,000m and worsen with altitude gain. Mild AMS (headache, slight nausea, fatigue): common and not immediately dangerous — rest, hydration, and not ascending further for 24 hours usually resolves mild AMS. Moderate AMS (persistent headache not relieved by paracetamol, significant nausea, severe fatigue): the trigger for descent — do not continue ascending with moderate AMS. Severe AMS / HACE (High Altitude Cerebral Edema): confusion, loss of coordination (ataxia — walk a straight line; inability to walk heel-to-toe without deviation is the field test), extreme fatigue, very severe headache — requires immediate descent and emergency oxygen administration. HAPE (High Altitude Pulmonary Edema): shortness of breath at rest, persistent cough (may produce pink or blood-tinged froth), cyanosis (blue lips/fingertips) — a medical emergency requiring immediate descent; the most dangerous altitude illness on Kilimanjaro with the highest mortality if not immediately evacuated. HAPE can progress from mild symptoms to incapacitation in 6–12 hours.
Diamox (Acetazolamide): Evidence and Dosing
Acetazolamide (brand name Diamox) is a carbonic anhydrase inhibitor that accelerates the blood chemistry changes of acclimatisation — it makes the blood slightly more acidic, which triggers increased breathing rate, effectively starting the acclimatisation process earlier. The evidence for Diamox in preventing AMS is strong: a Cochrane review of multiple randomised controlled trials found that Diamox at 250–375mg per day reduces AMS incidence by approximately 70% compared to placebo. Recommended dosing for Kilimanjaro: 125mg twice daily (250mg total per day) beginning 1–2 days before entering altitude (at the start of the trek for most routes) and continuing until 2 days after reaching the maximum altitude. Some physicians recommend 250mg twice daily (500mg total) for the summit night specifically. Side effects: diuresis (increased urination — this is expected, not a problem; stay hydrated), tingling in fingers and toes (carbonic anhydrase inhibition effect, harmless and transient), and occasionally nausea. The drug is a sulfonamide-related compound — those with sulfa drug allergy should not take it without physician consultation. Obtain on prescription before departure — Diamox is available in Moshi and Arusha pharmacies but sourcing in advance avoids uncertainty.
Acclimatisation Strategy by Route
Route selection is the single most important altitude safety decision. The Marangu Route (5 days standard) ascends too quickly — the 5-day Marangu has the lowest summit success rate (approximately 40–50%) and highest AMS rate of any Kilimanjaro route. The Lemosho Route (7–8 days) is the best acclimatisation option — the “high camp and sleep low” profile of the Lemosho with its rest day at Shira Plateau (3,840m) and gradual eastern approach from Barafu gives the most time for physiological adaptation. The “climb high, sleep low” principle is embedded in every Kilimanjaro route in the form of the Crater Camp or summit night — the camp at 4,700–4,800m is used as the summit launch point, with trekkers descending after summiting. 7+ day routes on any route increase summit success rate significantly over 5–6 day routes for the same age and fitness level.
Turnaround Criteria: When to Descend
The most important safety decision on Kilimanjaro is the turnaround decision — and it is often the hardest because summit fever (the psychological pressure to continue regardless of symptoms) is a documented factor in Kilimanjaro deaths. Clear turnaround criteria: any loss of coordination (the straight-line walk test) — descend immediately; shortness of breath at rest — descend immediately; HACE or HAPE symptoms — descend immediately and as fast as possible. Moderate AMS at the summit camp the night before the summit attempt — do not proceed; rest and descend to a lower camp for 24 hours then reassess. Severe headache unresponsive to paracetamol at the summit camp — do not proceed. The guide’s turnaround decision: a licensed Kilimanjaro guide (KINAPA-certified) has legal authority to require a trekker to descend — cooperate with this decision even if you feel you could continue. The guide carries the emergency oxygen cylinder and the Gamow bag (a portable hyperbaric chamber used for emergency altitude illness treatment) for this purpose.
