While much of the focus on legal drugs - also known as novel psychoactive substances (NPS) - has been on their use by young people, a survey has found established users of drugs such as heroin are also using them.
David Liddell, director of the Scottish Drugs Forum (SDF), said: "What we are also seeing is the use of NPS among the existing problematic drug users. This is now very much a part of the drug culture within wider society, but within the problematic drug-using population as well."
The SDF will present the survey findings at a conference on new drug trends in Scotland taking place in Glasgow on Tuesday.
Liddell pointed out it was often unknown what "legal highs" contained and the use of the drugs could destabilise addicts on methadone. He added: "If people are injecting stimulant drugs they could be doing that very frequently, which then has all the knock-on issues of blood-borne virus infection, particularly hepatitis C but also HIV and hepatitis B."
Efforts have been made to ban some "legal highs", such as mephedrone - also known as meow-meow - which became a class B drug in 2010. But consultant psychiatrist in addiction Dr Malcolm Bruce, who will speak at the conference, argued such bans were placing drug users at increased risk.
He said: "Having chosen to use drugs, it would be much better (for users) to use cannabis and run the risk of breaking the law than ... buying what is sold as a legal compound - which may be much more dangerous to them."
Community Safety Minister Roseanna Cunningham, who will address the conference, said: "We need to challenge the myth that legal equals safe."