People on methadone programmes are as likely as other addicts to use heroin and commit crime, according to a study.
The research, involving Glasgow University, said those using methadone took heroin on a fewer amount of days, but they committed just as much crime.
The Tories said Scotland was over-dependant on methadone and last month claimed annual costs had risen by 19%.
The Scottish Government said it aimed to provide the right support rather than promote one type of treatment.
However, one of the authors of the report, Prof Neil McKeganey, of Glasgow University's Centre for Drug Misuse Research, said it raised a question about the "gargantuan" size of Scotland's "£25m" methadone programme.
"One of the major drivers for the methadone programme in Scotland has been the belief that by providing methadone there will be a substantial reduction in the crimes addicts commit," he said.
"What this research has identified is that that is not being borne out.
"That must be a worry because that was often presented as one of the reasons why Scotland has encouraged the development of such a wide-ranging methadone programme."
Prof McKeganey said criminality among addicts was linked to more things than just drug use.
"If they've established criminality as part of their lifestyle then there is a strong impetus for that to continue," he said.
Scottish Conservative leader Annabel Goldie said the new analysis shattered one of the alleged benefits of methadone programmes.
She said the research was "another wake-up call to those who insist that the wholesale parking of addicts on a legal drug is preferable to leaving them on an illegal one".
A Scottish Government spokeswoman said the national drugs strategy, launched in May, recognised the need to help people move on from methadone through other treatments.
"Different people with different circumstances inevitably means different routes to recovery," she said.
"The challenge is to make sure that the right support is available at the time it is needed, rather than promoting one particular type of treatment."
Researchers looked at the behaviour of more than 400 drug users, including 68 who were using methadone at the start.
Source - BBC http://news.bbc.co.uk/1/hi/scotland/7781641.stm