Climbers scaling Mount Kilimanjaro are taking unnecessary risks with their health, experts have warned.
Travel firms have seen an increase in bookings following the successful summit by nine celebrities for last year's Comic Relief campaign.
But Edinburgh University scientists warned many climbing Africa's tallest peak "know little or nothing" about high altitude, which can be fatal.
Scientists camped out to test altitude sickness in more than 200 climbers.
The researchers camped for three weeks on the mountain at a height of 4,730m - not far below the 5,895m summit.
They assessed climbers using the Lake Louise consensus scoring system, which records symptoms such as headache, sickness and fatigue.
The academics found almost half, or 47%, of those who had climbed Kilimanjaro, were suffering from altitude sickness before they reached the summit and most were ascending too high, too quickly.
Signs of sickness include vomiting, headaches, difficulty sleeping and sometimes problems with co-ordination.
Effects can be felt from as low as 2,500m above sea level and 75% of people will have mild symptoms at 3,000m or higher, the study said.
Reserachers said the best way to acclimatise was to climb slowly and some trekkers incorporate acclimatisation rest days.
Some also opt for anti-sickness drugs, although there is controversy in the mountaineering community about whether they are effective.
However, the experts found that neither altitude-sickness drugs nor rest days had a major effect on whether people got ill.
They concluded that climbers were going up so rapidly, the drugs could not protect against the harmful effects of altitude.
It also did not matter which route people took, suggesting "the rate of ascent on any itinerary is sufficiently fast to cause acute mountain sickness in a large proportion of climbers".
However, climbers who had managed to acclimatise beforehand on nearby high mountains were less likely to suffer from sickness, the research said.
Stewart Jackson, who conducted the study, published in the journal High Altitude Medicine and Biology, said: "We found that many climbers knew little or nothing about altitude sickness and did not have previous experience of being at high altitude.
"This research emphasises the need to increase awareness of the risks of altitude sickness and the importance of taking your time to acclimatise.
"Undertaking an acclimatisation trek before attempting to summit Mount Kilimanjaro offers climbers the best chance of a safe, successful summit."
Severe altitude sickness can lead to serious complications, including shortness of breath at rest, inability to walk, decreasing mental alertness and a build up of fluid on the lungs which can result in a "gurgling" sound when breathing.
People with these symptoms must descend the mountain quickly or risk dying.
High altitude cerebral oedema and high altitude pulmonary oedema are rare, but deadly. In both cases, the lack of oxygen results in leakage of fluid into either the lungs or the brain.