Acute Mountain Sickness (AMS), or as now referred to as “altitude illness” is the major concern with climbing Kilimanjaro.
We recommend that our climbers consider taking acetazolamide (Diamox) to help them acclimatize, especially on shorter treks (7 days or less). Diamox is well-studied and proven safe (barring an allergy to Sulfa drugs) as a prophylaxis and as a treatment for AMS. It increases respiration and thus increases the amount of oxygen to the brain and other vital organs, especially during sleep. It mimics what your body would be doing after a few days at altitude, and it promotes better sleep.
The only drawback is its diuretic effect, and thus the need to stay well-hydrated at all times. The Center for Disease Control (the CDC) have provided on their website a comprehensive overview and diagnosis, prevention and treatment of AMS, including tables assessing risks and dosage recommendations for Diamox and other commonly prescribed and used altitude meds: See:http://wwwnc.cdc.gov/travel/