Heroin and cocaine addicts on the government's treatment programme are being given drugs as a reward for clean urine samples, the BBC has learned.
The National Treatment Agency (NTA), which runs the £500m-a-year scheme, admits the practice is "unethical".
Its own survey of almost 200 clinics in England found users were being offered extra methadone, a heroin substitute, or anti-depressants for good behaviour.
Health Minister Dawn Primarolo has asked for a report into the survey.
She said the survey had raised "very serious issues".
She said: "It is unacceptable, unethical, it should not happen that prescription drugs and doses are used, or suggested that they should be used, as either incentives or withheld as sanctions as part of a treatment programme."
A third of clinics in the survey said users who produced a drug-free urine sample may be offered increased doses of heroin substitute as a reward - known as "contingency management".
A quarter admit that clients can choose the type of substitute drugs they want.
The survey also found clinicians offering antidepressants, cash vouchers or access to detox as a reward.
The NTA said offering drugs for anything other than clinical need was wrong and it wanted certain practices "squeezed out of the system".
The agency's chief executive Paul Hayes told the BBC: "One of the things that's important before we start rewarding people through things like contingency management is to make sure that we're doing it according to the best principles for drug treatment.
"There are a range of practices associated with drug misuse in this country that are not what we would want them to be."
He said the NTA was set up to not only expand the provision of drug treatment, but also to improve its quality.
He added: "It is entirely appropriate to prescribe other drugs alongside prescription drugs that are to deal with withdrawal. Not as a reward, which is why we wouldn't advocate it.
"What we would say is the dose people get ought to be determined by the individual's needs, not by whether or not they're co-operating with the regime.
"That's why the contingency management programme that we're thinking of introducing, based on American research, is going to be very different to the ad hoc rewards that operate in not very well managed services in this country at the moment."
Martin Barnes, chief executive of drug information charity DrugScope, said it was "appalling" to offer drugs as a reward.
"It is a complete distortion of the principles of 'contingency management'," he said.
"The practice is unethical, contrary to official guidance and creates potentially serious risks for the drug user."
Matthew Taylor, of the Royal Society of Arts, a think tank looking at how best to get addicts off drugs, said an overhaul of current policies was needed.
"I think the reality is that our drug strategy just isn't working," he told BBC One's Breakfast.
"Only a very small proportion of those people who are put through drug detoxification successfully complete the programme, and even when people do successfully complete the programme they revert to drug use very quickly.
"So we need a different approach, and the fact that some people feel that they need to incentivise drug users with other drugs in order to keep them off illegal drugs is, I think, part of that general problem."
Dr Michael Ross, former clinical director of Bradford's drug dependency service, said drug addicts needed to be self-motivated to achieve results.
"The idea of bribing the patient to achieve a result which wasn't actually something they felt important is quite abhorrent," he said.
The drugs treatment project is the centrepiece of government strategy.
Only about 6% of users on the programme leave free of drugs each year. However, there is evidence that giving addicts access to services can reduce crime and improve health even if they continue to take drugs.
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