Police are continuing to investigate how South Sydney players Dylan Walker and Aaron Gray obtained prescription drugs in a move that raises the prospect of supply charges being laid.

The Rabbitohs pair were rushed to hospital in a critical condition last month after overdosing on painkillers, including tramadol.

The NRL's integrity unit is investigating how they obtained the drug, but because no illicit or performance-enhancing substances were involved, it was thought police had dropped off the matter. However, the case file remains open as authorities attempt to determine how the drugs were sourced.

It has been suggested the painkillers were not prescribed by club doctors after Walker and Gray underwent surgeries for hand and knee problems respectively.

Police are also continuing to investigate an incident in which Canterbury fans clashed with police during the side's elimination final loss to the Sydney Roosters last month.

South Sydney have agreed for their players to be used to trial hair-follicle testing for prescription drugs after Fairfax Media reported Walker and Gray had not been prescribed one of the two painkillers that led to their overdose.

The NRL has conducted more than 2000 tests after becoming the first Australian body to test for prescription drugs, with results suggesting the problem is not widespread. However, anecdotal evidence suggests otherwise.

The New Zealand Rugby League acknowledged, after conducting an investigation, that the misuse of prescription drugs with energy drinks contributed to the Kiwis' capitulation in the last World Cup. There was also speculation the Queensland Origin side was adversely affected by the practice during the 2009 series, while Russell Packer openly spoke of his misuse of prescription medications. League journeyman Ryan Tandy admitted to misusing them in his memoirs and died from an overdose.

Dr Peter Larkins, a specialist sports and exercise physician and former national president of Sports Medicine Australia, said problems can occur "when the club doctor is left out of the loop".

"This is a narcotic and narcotics kill people. You can die from an overdose of narcotics," Larkins said. "If you suddenly get some free narcotics through the prescription system, be aware that narcotics are associated with side effects in overdoses, in the same way street narcotics are.

"People die of overdoses from narcotics in the street every day. The fact players have end of season surgery and go on narcotics for pain relief is not new. Where the system falls down in ticking all the boxes is when sometimes club doctors aren't necessarily involved with the players at end of season surgery time. It's usually surgeons and doctors not completely on top of all the rules and regulations [are involved]."

Larkins didn't believe the abuse of prescription drugs was rife in the NRL or other Australian sporting codes, but said the Rabbitohs' incident was a timely reminder to all athletes.

"I don't think it's out of control because we haven't had a lot of cases about this in the last few seasons repeatedly," he said.

"What doesn't smell right [about this case] is the instructions they were given to use it, that they ignored the instructions or they have had something with it that's reacted with it."