Some drug addicts in Scotland who take the heroin substitute methadone under supervision are reselling it later after spitting it out.
A BBC investigation found the practice known as "spit-meth" helps the addict pay for heroin.
Former Strathclyde police officer Tom Wilson said addicts devise techniques whereby they hide the drug under their tongue or at the back of their throat.
Once out of the pharmacy they spit it into a container and sell it.
The investigation also found that the body which represents Scottish pharmacists wants the law to be changed to try to reduce the number of addicts receiving methadone.
Currently, only doctors can change methadone prescriptions.
Lyndon Braddick, from the Royal Pharmaceutical Society, said pharmacists were best able to judge whether an addict might benefit from a lower dose.
He said this might help reduce addicts' reliance on the drug to zero.
Mr Braddick, director of the society in Scotland, said: "It would help to reduce the dosages of methadone that people were on and help them and support them towards a lifestyle which is less dependent on taking methadone and on taking opiates, because it would put the patient in greater control of their own treatment."
He added that there could be benefits in a more collaborative approach involving all the professionals who are working with a particular addict, including doctors, social workers and pharmacists.
For at least the last two decades, the prescription of methadone to help drug users beat their addictions has been a growing phenomenon.
Latest available figures for Scotland indicate there has been a 19% increase in methadone prescriptions in the five years between 2004 and 2008 and the official estimate is that at least 22,000 people, probably more, are being prescribed the drug.
The problem with methadone, according to one point of view, is that it replaces an illegal addiction with a legal one. And in some cases it doesn't even do that.
Many ex-users say they only used methadone when they couldn't get heroin, others that they sold their prescription in order to get money for illegal drugs.
One way to sell on the substance is to spit it out after receiving it from the pharmacist.
Ex-policeman Mr Wilson explained: "In practice we would see the addict on the methadone programme go into the pharmacy, he would take the methadone into his mouth, the pharmacist should, and in most cases does, ask the addict to open his mouth and check his mouth to make sure his mouth is empty.
"Now, what they have done is they have developed all sorts of means of hiding the methadone in their mouth, under their tongue, just as a general liquid throughout their mouth - they half swallow it, put it to the back of their throat and it looks as if it's clear.
"When they come out they'll have a container outside and they would just gargle it up and spit it out - along with their phlegm and whatever else. They would then give it to their 'customers' who would then just drink it."
One former addict, Billy, who has now been in a recovery programme for four months, said he was on methadone for 21 years and during that time he was still using illegal drugs like heroin.
He added: "I used to go and pick up 600ml or maybe more and it would go in the fridge unsupervised.
"And then I would go and sell my methadone and I would go and buy heroin with it. Sometimes I had row after row of bottles, saving it up. I used to save it up and sell it."
Billy said it was only when he enrolled on a residential recovery programme, run by the organisation Phoenix Futures, that he was able to start getting his life back to normal.
However, many experts on drug abuse insist that methadone plays a crucial role in reducing the harm caused by addiction.
They say that without it, more people would be at risk of disease and overdoses because of the dangers inherent in using illegal drugs. Their lives would be more chaotic and they would be more likely to commit crime.
A spokesperson for the British Medical Association Scotland said the idea of giving pharmacists more autonomy required careful consideration because managing patients on methadone required a "multi-faceted approach".
The Scottish government is now encouraging new strategies to help addicts get their lives back to normal.
The Scottish Drugs Recovery Forum is an initiative set up to promote the idea that a full recovery from addiction is achievable.
It has carried out research which suggests filling addicts' lives with meaningful activities and relationships is the best way to help them beat their addictions.
You can hear James Shaw's report on methadone on BBC Radio 5 live Breakfast 0600-0900 BST.