At last, a politician saying something sensible about drugs and addiction

This is a significant moment. If nothing else, Oliver Letwin has created the space for a saner and more nuanced debate

Not all of the proposed policy would work but there was more sense in it than a lot of what we've heard about drugs down the years. The Tory Home Affairs spokesman, Oliver Letwin, is offering rehab and not retribution, tough love rather than a thrashing. At first glance it looks like the familiar carrot and stick, but Mr Letwin has shown more imagination than any of his predecessors. The fact that he eschewed the traditional "hang 'em, flog 'em" approach so beloved of past Tory conferences represents more than an attempt to re-brand the party's image. Mr Letwin's ideas about rehabilitation rather than jail are part of a much deeper realisation: addiction is an illness and you cannot criminalise it out of existence.

In different ways the Labour Party and the Liberal Democrats have been saying much the same thing. But now Mr Letwin is giving sinew to the philosophy with some practical ideas. The sad thing is that the politicians are several years, if not decades, behind the experts. In this debate we should be clear that we are talking predominantly about the children of the poor. Politicians will never quite say it of course. It might sound patronising or, more likely, be taken as a symptom of their collective failure to deal with poverty.

Rich people's children get addicted too but they don't end up in court nearly half as often. For them rehab is an option of first resort. Nor is Mr Letwin referring to the cocaine-addicted high-fliers who can afford expensive treatment here or in America. Still, the shadow Home Secretary's speech shows a refreshing willingness to recognise that addicts are products of something more complicated than moral weakness or inherent wickedness. Having realised this, the imperative is to steer policy on addiction away from the prisons and into a safe place.

The latter is an expression frequently used to describe rehab. In theory this should be a place where there are no drugs or booze, where there is food and warmth (two of the first things to go when serious addiction sets in) and where counsellors – often former addicts – are on hand to guide the junkie towards a clean and sober life. But here is the first point at which the politicians need to take a reality check.

When you promise to send addicts to rehab you must understand the cost. Any kind of worthwhile rehab costs serious money to run. At the minimum, addicts need 28 days to get on the road to recovery; in many cases, we are talking about much longer-term care, of between three to six months. Any new centres will effectively have to be run as small hospitals with detox units, medical staff and trained counsellors. With a cash-strapped NHS, one can already predict the storm of howling on the right: why are addicts being mollycoddled when the elderly can't get hip replacement operations? Mr Letwin will not only need to convince the Treasury and battle with the Department of Health, he will need to be tough with many of his own party's supporters.

After coming out of rehab, a large proportion of recovering addicts will need to enter "half-way house" accommodation if they are to stand any chance of staying clean. This means thousands of houses around the country staffed by trained counsellors. Again Mr Letwin would have to go with his hand outstretched to the Treasury and face potential aggravation from residents who don't want recovering addicts in their back yards. Any politician willing to fight these battles could make a dramatic impact on the levels of drug-related crime and on the already horrendous cost to the NHS of drug-related illness, not to mention the human misery caused by addiction. One way or another, society pays when people become addicted to hard drugs.

To anybody familiar with addiction there are a couple of obvious weaknesses in Mr Letwin's strategy. While he is perfectly correct to identify rehab as the way forward, it is more doubtful whether offering addicts the choice of jail or rehab is sensible. If addiction is an illness then it makes no sense to threaten jail for possessing a drug. The choice on offer would work out something like this: if you don't go to rehab we will send you to a jail where you can still get drugs and carry on being an addict.

Certainly if an addict commits a crime against another person they should be liable to prosecution. For crimes like murder and serious assaults, jail has to be the only option. The addicts who mug and beat cannot hide behind the mantra of "I'm a victim too". But for the lesser crimes that addicts are charged with when they first enter the system – shoplifting, burglary, credit-card fraud – magistrates should be encouraged not to offer a choice but to send addicts straight to rehab. It may just stop them from graduating to the place where they mug elderly women for their pensions.

There also needs to be a strategy that finds the addicts before they find the criminal justice system. Oliver Letwin talks of "identifying" addicts. But how is this to be done? Relying on parents who, in many cases, have fallen foul of the police themselves isn't a realistic option. Nor is it right or practical to depend on neighbours to snoop on who is snorting or injecting what. The problem might be resolved by concentrating resources in the place where the problem starts. If a government is willing to get counsellors on to the council estates where many of the addicts live, there is a far greater chance of them being persuaded into treatment centres before they ever come into contact with the courts. So set up drop-in centres and send in counsellors who are themselves recovered addicts.

The most glaring omission from Mr Letwin's speech – and from the one delivered by the Home Secretary, David Blunkett, the previous week – concerns the most dangerous drug in Britain. Neither the minister nor the man who would be minister saw fit to refer to alcohol. In Simon Hughes' speech to the Liberal Democrat conference in Brighton, alcohol was mentioned twice but not in any detail. Is there a lack of awareness across the political spectrum or is this a case of group denial? Just this week the charity Alcohol Concern revealed that unsafe drinking among young women doubled in the last decade. More than 30,000 deaths each year are attributable to alcohol misuse. When it comes to crime the figures are astonishing: alcohol is a factor in 60-70 per cent of murders, 75 per cent of stabbings and 50 per cent of fights and domestic assaults. Every year there are almost 1.5 million victims of violent attacks committed by people under the influence of booze. Focusing on drugs and drug-related crime alone is wrong-headed.

All that being said, the Letwin speech represents a positive and significant moment. It is probably unrealistic to hope for a cross-party approach to this problem, but if nothing else Oliver Letwin has created the space for a saner and more nuanced debate. All of the politicians need to be honest and tell us that tackling this problem will cost and may not deliver results for several years. But they might add that current policies have cost us the earth and taken decades to achieve nothing.

The writer is a BBC Special Correspondent