If you are planning to climb Everest, it may be prudent to pack some ibuprofen, research suggests.
A study has shown that the drug, commonly taken to relieve pain and inflammation, reduces the risk of altitude sickness.
Symptoms of the condition include headache, fatigue, dizziness, nausea, and vomiting.
Left untreated, it can lead to potentially fatal water retention in the brain.
Scientists in the US studied 58 men and 28 women who hiked to an altitude of 12,570 feet in the White Mountains of California.
Of those treated with three doses of ibuprofen on the way, 43% suffered from altitude sickness, also known as "acute mountain sickness" (AMS). In comparison, 69% of hikers who were given a non-active dummy drug experienced symptoms.
Less severe symptoms were generally reported in those affected while taking ibuprofen, although this was not statistically significant.
The findings, published in Annals of Emergency Medicine, could prove useful for people holidaying in high places, say the researchers.
"You don't want to feel horrible for 15 to 20% of your vacation," said lead scientist Dr Grant Lipman, from Stanford University. "Ibuprofen could be a way to prevent AMS in a significant number of the tens of millions of people who travel to high altitudes each year."
Two other drugs prescribed to prevent altitude sickness, acetazolamide and dexamethasone, can have severe side effects, the researchers pointed out.
They wrote: "We suggest that availability alone makes ibuprofen an appealing drug for individuals who travel to high altitudes. In addition, ibuprofen was effective when taken six hours before ascent, in contrast to acetazolamide, whose recommendations include that it be started the day before travel to high altitude."
However, the benefits of taking more than 600 milligrams of ibuprofen to stave off altitude sickness had to be balanced against the possible risk of gastrointestinal and kidney problems in people who are dehydrated.