The first astronauts heading to Mars will venture further than any human before them, and should any of them encounter medical problems on their journey – perhaps “cosmic kidney disease” or a bacterial infection – they will be almost entirely alone to deal with them. Adding to those challenges, a new study has identified another problem; medications brought along on the mission will expire before the astronauts return to Earth. 

On the International Space Station (ISS), astronauts have access to a variety of medications that can be replenished by resupply missions if depleted or expired. However, a trip to Mars will take around nine months each way, with an extra three months spent on the Red Planet waiting for Earth and Mars to be in a suitable position for the return journey. That's 21 months in total, with no new supplies.

NASA does not routinely disclose medication used on the ISS, but the new study's authors used a Freedom of Information Act Request to obtain a list, assuming that a Mars-bound mission would have a similar stock. The researchers, from the Duke University School of Medicine, then assessed how long each drug would be expected to last in its original packaging. This too raises a problem, as shelf-lives are based on the drugs being kept in their original packaging, but drugs sent to the ISS are repackaged before launch.

On top of that, we don't know how long these drugs last in space. It's possible that they will degrade more quickly due to radiation.

The good news is that the shortest shelf life (for an ophthalmic lubricant) is 18 months, posing a "minimal risk" for extended missions to the Moon – but for longer missions, astronauts could see their drugs become less effective.

"Using the maximum labeled shelf-life across all sources for each medication, we found that 14 medications will expire by 24 months: one ophthalmic lubricant, one advanced life support medication, one anaphylaxis treatment medication, one benzodiazepine, one antiangina medication, two corticosteroids, one local anesthetic, one topical urinary jelly, two antibiotics, one antipsychotic, one inhaler, and one ear wax removal medication," the team writes in their study. "Furthermore, over half [...] of the entire 2023 ISS formulary will expire by 36 months."

Using more conservative estimates of efficacy based on each medication’s minimum labeled shelf-life, 97.8 percent of medications taken aboard the ISS expire within 36 months.

Drug manufacturers assure the effectiveness of their medication up to the expiration date, but beyond that, they may become less effective. 

“It doesn’t necessarily mean the medicines won't work, but in the same way you shouldn’t take expired medications you have lying around at home, space exploration agencies will need to plan on expired medications being less effective,” Daniel Buckland, assistant professor in the Department of Emergency Medicine at the Duke University School of Medicine and aerospace medicine researcher said in a statement.

"Ultimately, those responsible for the health of spaceflight crews will have to find ways to extend the expiration of medications to the complete mission duration," the team concluded, "or accept the elevated risk associated with administration of an expired medication."

The study is published in the journal npj Microgravity.