People who buy the party drug ketamine are just as likely to end up with another substance, according to Canberra's pill-testing service.
Results from the first five months of the CanTEST trial show that only 53 per cent of drugs thought to be ketamine actually were.
That trend continued in the service's latest monthly results, which covered the Spilt Milk music festival, when demand for confidential drug checks soared.
CanTEST analysed a record 145 drug samples in that period. Unusually, the vast majority of substances were what users expected them to be.
The exception was ketamine, an increasingly popular illicit drug that is used legally by veterinarians and in hospitals as a sedative.
Directions Health Services, which is running the pill-testing trial, said six of the 12 "ketamine" samples were different drugs.
Four were the recently discovered mystery substance nicknamed "CanKet", about which little is known. One sample was MDMA, which is commonly called ecstasy.
Over the past five months, Canberrans mostly tested drugs they thought were MDMA, cocaine or ketamine.
Directions acting chief executive Stephanie Stephens said the service had detected a lot of variability in both the contents and potency of the substances described as ketamine.
"It is quite a popular drug at the moment," she said.
"And so it's really important to us that we're able to keep testing this and help people to understand its contents."
Ms Stephens said synthetic substances and research chemicals were often used as substitutes.
"The synthetics are quite strong at times, and their impact isn't well understood abroad or in Australia," she said.
"This is a conversation that we've been having for years around cocaine and other drugs as well.
"They're cut with so many things, but with those used to replace or cut ketamine … very little is known about them."
Swamped service needs more staff: review
The ACT government recently extended the CanTEST trial, which is Australia's only free, legal, pill-testing service.
It allows people to anonymously get substances checked and receive health advice while they await results.
However, an interim evaluation warned the service needed more resources, as it had struggled to provide enough testing before large dance and music festivals.
Ms Stephens said the service had extended its opening hours in the lead-up to Spilt Milk but still turned people away "because demand was so huge".
The evaluation recommended CanTEST be funded to employ two analytical chemists, rather than just one.
Pill testing remains a controversial policy in Australia. All governments except the ACT's oppose it, arguing it facilitates drug use.
However, research based on European services suggests that, over time, pill testing can improve the safety of illicit drug markets by discouraging the sale of dangerous substances.
University of New South Wales professor Alison Ritter has also argued that, where the services are available, checked substances "started to correspond to the expected components".
CanTEST says it is too early to draw conclusions about the effects of its service on the drug trade.
