Medicines do not appear to degrade faster in the zero-gravity atmosphere on the International Space Station (ISS) and this does not differ from what is typically seen on the Earth, researchers report.
While the orbiting international laboratory is regularly re-supplied with medicines to replace those which have passed their expiry date, this may not be possible on exploration missions that travel to more distant points.
On the Earth, medicines degrade over time, particularly when exposed to light, oxygen or humidity.
Temperature and humid conditions on board the ISS are generally within ideal ranges for medicine storage on the Earth.
"Till now, there has been little evidence of how medicines might react to factors unique to spaceflight such as micro-gravity and constant exposure to elevated radiation levels," said the team from the Centre for Space Medicine and department of pharmacology at Baylor College of Medicine in the US.
Lead researcher Virginia Wotring analysed nine medications which had been stocked on the ISS and returned to the Earth unused after 550 days of storage in spaceflight.
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The medications included sleeping aids, pain relievers, antihistamines/decongestants, an antidiarrhoeal and an alertness drug.
The medicines were returned to the Earth and, upon arrival, they were kept under controlled conditions till analysis three-five months later.
The researchers measured the quantity of active ingredients and degradation products present in the medicines.
One medication met requirements five months after its expiration date. Four of the nine drugs were still viable up to eight months after officially expiring.
Three other medications met the guidelines when they were tested three months before their expiry date.
"The findings suggest that further research is necessary before planning long-term space flights -- such as missions to Mars -- because missions like this will not have the opportunity to restock medicines in the way that the ISS can," the authors noted.
The study appeared in The AAPS Journal, an official journal of the American Association of Pharmaceutical Scientists.