Over the past 12 years, the government has spent billions on tackling problem drug use. Has it been money well spent? The official answer, we learn today, is "haven't the foggiest".

Today an influential all-party Commons committee said it was "unacceptable" that the government has never bothered to evaluate the measures on which huge sums have been lavished as part of its drugs strategy for England and Wales (see Tackling problem drug use [647KB PDF] ).

In a short but pithy report, the MPs simply state that the Home Office "does not know how to most effectively tackle problem drug use".

The report concludes that:

"The Government spends £1.2 billion a year on measures aimed at tackling problem drug use, yet does not know what overall effect this spending is having."

Why? The Treasury demands that value for money audits are conducted on almost every aspect of public spending - except, it seems, when dealing with the "evil" of drugs. On this, it would seem, ministers have preferred to rely on instinct.

Twelve years after launching the first drugs strategy, the Home Office has now agreed "to produce an overall framework to evaluate and report on the value for money achieved from the strategy, with initial results from late 2011".

What will be interesting is whether the demand for evidence in this area will prompt ministers to consider ideas which, in Britain at least, are regarded as politically off-limits.

The billions which have been spent on the government "strategy" are based on the belief that the way to reduce drug-related crime is to get problem users into treatment. But the Home Office has been forced to admit that the theory is no more than that - an unproven theory.

"The Department said that it did not know whether the strategy had reduced the £13.9 billion cost of crimes committed by problem drug users and it could not prove a causal link between the measures in the strategy and the levels of offending by problem drug users."

This is a far cry from the mantra which for years underpinned the entire drugs strategy (see Tackling drugs, changing lives [289KB PDF] ):

"Every £1 spent on treatment saves at least £9.50 in crime and health costs."

It was a no-brainer for ministers desperate for an answer to the drugs crisis and the phrase popped up all over the place.

But even while home secretaries were using the line to justify massive investment in the drug treatment programme, internal government documents admitted the claim was "out of date".

Many would argue that drug treatment is clearly better than no drug treatment and, for many people in crisis with their addiction, the support and help offered under the NTA scheme can be a life-saver.

It must also be true that, if someone is committing crime to fund a drug habit, an intervention which reduces or eradicates that addiction will probably reduce their crime.

But are we spending the money on the right kind of treatment for the right people? And, perhaps more importantly, is the answer to almost £14bn worth of drug-related crime simply more treatment or something more radical?

The National Treatment Agency (NTA), which runs the treatment programme for drug users in England and Wales, was asked whether their work cut crime.

NTA Chief Executive Paul Hayes replied that it had produced "very significant reductions in crime, and that is what has justified the huge increase in investment in drug treatment we have seen since 2001".

However Sir David Normington, the top civil servant at the Home Office, was asked for proof that recent falls in acquisitive crime could be attributed to treating addicts. "I cannot prove an absolute causal link but it is a fair bet", he replied.

When pressed, the NTA acknowledged that "over one-quarter of problem drug users showed a sharp increase in the volume of offending after entering treatment through the Drugs Intervention Programme".

Interestingly, for this group of largely heroin addicts, the Commons committee latched on to the idea of giving them free heroin. Here is the exchange between committee member Ian Davidson MP and Sir David:

Mr Davidson: "Have you considered giving them free drugs as a means of cutting crime in order to make everybody else's lives better?"

Sir David Normington: "There are those who think that should happen. That, of course, is absolutely not the Government's policy. I think it is a sort of counsel of despair because it does not take you anywhere. It means that you leave these people on drugs forever."

Mr Davidson: "It is not a counsel of despair for the people who are living beside them, whose houses are getting broken into, with respect."

The report also asserts that "measures to reduce problem drug use by young people have had limited impact".

The most recent strategy published in 2008, the MPs note, "reported that the prevalence of Class A drug use by young people has stayed relatively unchanged since the first strategy in 1998".

Another example of how hard it has proved to reduce drug harms is the recent rise in the number of people who die from illegal substances.

"The number of deaths among problem drug users has increased over the last five years to 1,620 in 2008-09", the report's authors point out, although the NTA suggested to them that "there would have been 2,500 drug related deaths in that year if drug treatment had not been increased over this time."

We are not to know what would have happened if the strategy had not existed, of course, but this is yet another report which questions whether our approach to drugs is actually the best way forward.

It is a vital debate for Britain and yet, as I suggested here yesterday, I doubt it will be discussed with any vigour during the election campaign.