ONE night in the early 1970s I was stranded in a farmhouse in the wilds of Aberdeenshire.

I had been at a party and missed my lift back to civilisation. There was nothing for it but to settle down and wait for daylight to arrive. Sleep, however, proved elusive, because my fellow party-goers, each of whom was in a deep, cannabis-induced dwam, could not be persuaded to take Hotel California off the turntable. So for me the Eagles and weed are umbilically associated, and not in a good way.

Even then I knew intuitively that drug-taking was not conducive to mental agility. Friends of mine smoked cannabis continually and spent their days floating around the student union like Victorian spinsters prone to swooning. They found it hard to focus let alone concentrate, and, being linguistically challenged, often used the word "man" in their conversation as if it were a punctuation mark. Attendance at lectures was sporadic and several of them dropped by the wayside, doomed, one suspects, to perpetual hippiedom. Where are they now? Kathmandu, possibly, or, more likely, Limbo.

It will come as little consolation to them to learn that what one always suspected is true: smoking cannabis does impact on its users' IQ. A study of 1000 cannabis users, by researchers in Dunedin, New Zealand, which claims the dubious distinction of being the first city to hold a Global Marijuana March, has concluded that the earlier you smoke cannabis, the more harm it causes the brain. Moreover, the more you smoke, the greater the impact on your IQ. For the researchers, it has confirmed what many already thought they knew, that cannabis users are likely to under-perform in the real world, if, that is, they ever become a part of it.

Whether this will influence addicts remains to be seen. My guess is that Hotel California still has its fans who will remain loyal until they've puffed their last joint. What the new research should do, however, is spike any proposal to legalise cannabis which, in its present form, is a much more potent drug than that which many of my contemporaries inhaled. Not only is it addictive and incapacitating, it leads its users on to other, even more dangerous drugs. Indeed, far from legalising it, the police should be exercising the powers they already have, and clamping down on it instead of turning a blind eye.

Widespread use of such "recreational" drugs is a legacy of the 1960s, popularised by rock bands, the most influential role models in history. If the Beatles or the Rolling Stones were in favour who could deny themselves? LSD, said Lewis Yablonsky, a professor who studied hippies as others once did cannibals, offered "is the key to cosmic consciousness and universal unity". On the other hand, cannabis, when smoked communally, was a more basic staple of everyday life, helping to cement the "circle of friendly love".

When you were away with the fairies, which is exactly where many of the users were for much of the time, you could chunter about "flower power" to your heart's content without fear of anyone dismissing you as a heidbanger. Like every scene, the drugs one had its cult figures, most notably Timothy Leary. A Harvard academic with about as much common sense as Prince Harry, Leary gave scientific heft to the notion that drugs were the conduit to a good society.

Sex, needless to say, was at the heart of this flapdoodle, of which there was plenty. "The fact is," Leary told Playboy magazine, "that LSD is a specific cure for homosexuality," going on to illustrate his argument by telling how a lesbian was transformed into a heterosexual after she'd taken some acid.

Not surprisingly, therefore, drug use grew exponentially throughout the 1960s. In 1960, there were 235 cannabis convictions. By 1970, there were 7520. But then as now the figures give no clear indication of how many people used cannabis, which in certain parts of the country may currently be as high as 30%. Not all of them, of course, are addicted to it, but many are, countless of whom have seen their dreams of starring on University Challenge quite literally disappear in smoke.