Drugs policy is not about science, health or balancing of risks. It is all about politics and the careers of politicians. That seems to be the inescapable conclusion of the events which unfolded over the past week. And what is most amazing is the brazen manner in which politicians come out with this and still expect our gratitude and respect.

Drug and substance abuse is bad. We should not underestimate the harm that some drugs do; as doctors we see evidence of this on a day to day basis. As human beings we see it all too often in friends, family and colleagues. Most of us feel that society should take some sort of collective action in order to restrict the use of these harmful substances, in some way.

Most of us are also comfortable with the idea that these restrictions should be graded across a spectrum. So some drugs are freely available (such as caffeine), others should be restricted in some way (alcohol or tobacco) and others should be made unavailable. Those that are unavailable are also graded so that the penalties for using the more harmful are more severe than the penalties for the less harmful.

In an ideal world, then, the decision of how to grade any individual drug would be based on the harm the drug does, and the harm to casual users, to addicts and the vulnerable, and to society as a whole. Quantifying and balancing these risks is a difficult science, requiring a range of scientific and sociological disciplines to work together to seek an answer.

But after the scientists have done their job and quantified the risks, and assuming their findings are valid and correct, then surely the job of the politician would be to rubber-stamp them. But this is where the whole model falls apart, and where an almighty row has erupted. Because even when the scientists have measured the risks to the nth degree, the politicians still think that they know better. They want scientific advice and government policy to be two separate concepts. 

And while Alan Johnson wriggles in the limelight and takes the flak, Her Majesty’s opposition is ready to leap to his defence. Writing in The Times this week Lord Young, the former conservative minister, is anxious to defend the right of politicians to ignore advice whenever it suits them. “The Minister will have other considerations to take into account” is his explanation. He argues so forcefully that it almost sounds credible.

But what other considerations does the minister have to take into account in setting drugs policy? If the scientific advisors have done their job properly and thoroughly there should be no stone unturned. A moment’s thought and the “consideration” that Lord Young refers to becomes obvious. It is of course, the political implications, and the repercussions on the career of the minister and his government.

So there you have it. But why stop at drugs? This argument extends to every corner of government policy, including the running of the NHS. And what I find so astonishing about this affair is that there has been no pretence of a cover up story.

For those of us brought up to believe in the scientific method, and that somewhere out there lies the truth if only we could find it, this all comes as a bit of a shock. I always thought the people upstairs knew what they were doing, and were acting on the basis of the best information available to them. Scientists like to believe in the purity and absoluteness of scientific fact.

But maybe I’m just being a little naïve. Groucho Marx understood the problem much better than me. “I have principles”, he said. “And if you don’t like them, well I have others.”