So-called synthetic highs are becoming increasingly popular among recreational drug users in Australia, according to an annual survey of people who use psychostimulants like ecstasy.
It's a big worry, says chief investigator Dr Lucy Burns of the University of New South Wales (UNSW).
The main problems are that the contents of the drugs are unknown and the effects are unpredictable.
Although use by the general population is likely to be "very low", Dr Burns is worried about the growing popularity among users of psychostimulants.
"Almost half of people who take conventional drugs like ecstasy also use the new class of drugs or emerging psychoactive substances (EPS). It's a concern. There have been fatalities," says Dr Burns, who is presenting the survey results at a drug trends conference in Melbourne.
Her team surveys hundreds of drug users each year in an attempt to keep up to date with popularity, price and availability of illicit drugs.
The latest survey, conducted in state and territory capitals in early 2013, shows 44 per cent of regular drug users used an EPS - including synthetic cannabis - in the previous six months.
This is up from 40 per cent in 2012.
Although ecstasy is still the most popular drug after cannabis, Dr Burns says things have changed.
"It's a whole new world out there. We have something like 250 substances," she said.
"We really don't know what's in them. What people get over the internet is often not what they think they are getting.
"A very significant proportion of young Australians in the recreational drug scene are using them and the effects can be unpredictable."
The survey shows use of methamphetamines like speed and ice has fallen significantly among the people interviewed.
The use of hallucinogenic drugs like LSD and ketamine has increased significantly, although frequency of use is low.
Although ecstasy is the most common drug of choice among people in the survey, they use cannabis and tobacco more often than any other substance.
Their tobacco consumption has fallen, however.