Prescribed painkillers can have serious side effects if not taken as directed, says expert after overdose by footballers.

Prescribed painkillers like oxycodone can be dangerous if not taken as directed, warns an expert.

South Sydney players Dylan Walker and Aaron Gray are in hospital after suffering a suspected overdose of prescription drugs on Tuesday.

The Rabittohs rugby league duo were prescribed painkillers following post-season surgery last week, but were found vomiting by a mutual friend at Gray's Sydney apartment in the early hours of the morning.

Early reports suggested Walker had been prescribed painkillers oxycodone and tramadol.

Dr Suzanne Nielsen, from the National Drug and Alcohol Research Centre, told AAP both were opioid medications, commonly used in the treatment of pain.

If mixed with other medications or alcohol, or not taken in the prescribed doses, the effects can be dangerous and unpredictable.

"Opioids can affect people's breathing and that's where the overdose concern comes in," she said.

"They do have serious side effects and it's important for people to know they are strong medications.

"They need to be taken as directed."

While not commenting on the Souths case, Dr Nielsen said people may ingest higher doses because the initial dose didn't appear to work, or for other reasons.

"It is known they can cause feelings of euphoria, or pleasant feelings," she added.

The number of Australians being treated for dependence on codeine and oxycodone had trebled in the nine years from 2002, she said.