Astronauts on long space missions may not be able to take paracetamol to treat a headache or antibiotics to fight infection, a study has found.
Scientists at the Johnson Space Center have shown that the effectiveness of drugs declines more rapidly in space.
Continuous doses of radiation onboard spacecraft may be to blame, according to the study published in the AAPS Journal.
The authors said longer missions have increased the need for drugs in space.
On Earth, medication is typically designed to be stored for a couple of years from the manufacture date. They normally need to be kept in precise conditions, such as away from direct sunlight or in a cool, dry space.
The research team investigated whether the unique environment of space - including radiation, excessive vibrations, microgravity, a carbon dioxide rich environment and variations in humidity and temperature - affected drugs' effectiveness.
Four boxes of drugs, containing 35 different medications, were flown to the International Space Station.
Four identical boxes were kept in controlled conditions at the Johnson Space Center.
The boxes came back to Earth after varying lengths of time in space. One was there for just 13 days, whereas another spent 28 months on the space station.
The study concluded: "A number of formulations tested had a lower potency after storage in space with consistently higher numbers of formulations failing United States Pharmacopeia potency requirement after each storage period interval in space than on Earth.
"This reduction in potency of flight samples occurred sooner than the labelled expiration date for many formulations suggesting that storage conditions unique to the spacecraft environment may influence stability of pharmaceuticals in space".
Dr Colin Cable, science information adviser at the Royal Pharmaceutical Society, said: "On Earth, medicines are tested to assess the effects of, for example, temperature, moisture, oxygen and light, and are packaged and stored to ensure they remain stable and effective over their shelf life.
"Repackaging of medicines into containers that do not give the medicines the protection required to moisture, oxygen and light can have a detrimental effect on their stability."
He added that radiation was known to affect medicines, depending on the dose used.
"One potential benefit of keeping medicines in a Space Station is that the medicines will be exposed to a carbon dioxide-rich environment, this may help minimise the degradation of those medicines prone to oxidation, such as adrenaline, vitamin C and vitamin A."