Stilnox is a quick-acting drug for insomniacs. It's not a drug that would be ideal for athletes. And definitely not for fun, an expert says.

One of its side-effects is muscle weakness.

"That's not something you would think would be good for sporting people," says Dr Lynn Weekes, the head of Australia's national prescribing service NPS MedicineWise.

People who take sleeping pills for fun "are putting themselves in danger", she told AAP on Friday after James Magnussen and four of his freestyle relay teammates admitted taking the drug in a bonding session before the London Olympics.

People who take Stilnox usually feel the effects within 15 minutes. Its main use is to help a patient with insomnia fall asleep and stay asleep.

It is potentially habit-forming.

"It is not a medicine to play around with," says Dr Weekes.

"I would never recommend that someone takes someone else's prescription medicine. Most Stilnox is bought legally, but it is also sold on the streets."

Stilnox has significant hypnotic effects and some users have reported unexplained sleepwalking, sleep driving, binge eating while asleep, and performing other tasks while sleeping.

Research by NPS MedicineWise has found these events mostly occur the first time the drug is taken or within a few days of the initial dose.

An unusual feature of the drug is that sleeping people under its influence can sometimes carry out complex tasks and participate in question and answer sessions as if they are awake.

People who take the drug can still feel sleepy and hungover the day following a dose.

In Australia, Stilnox packs carry the following alert: "Warning: ... may be associated with unusual and potentially dangerous behaviours whilst apparently asleep. These have included sleep walking, driving motor vehicles and other bizarre behaviours ... You must not drink alcohol ...