Fewer people are smoking cannabis these days, down to 1.4 million from two million, they say. I say, if you believe that, you’re high.

Arrests, prosecutions and the issuing of ‘cannabis warnings’ might be down — but then, I’ve seen the police quite deliberately look away from dope smokers on the street.

Weed is everywhere. I’m sure of this, because the smell of the city has changed. A decade ago, as I cycled across town, the dominant scent was diesel. There were also wafts of tobacco from the fag-break gang and the odd drift of ground coffee.

Ten years later both the cigarettes and the diesel have faded. There’s the cartoonish smell of vaping, but as far as I’m concerned, the smell of London is now cannabis.

It hangs over Angel, Hoxton, Clerken-well, Covent Garden. I’ve smelled it on Holland Park Avenue and the Kings Road. It’s a depressing smell. Not exotic at all, just cloying and sad. Yesterday evening, a little waft on Exmouth Market, then the usual outside the park. I’m not sure I’ve had a dope-free cycle across town since sometime in 2017.

On the Embankment, most mornings, the cold air is cut with cannabis. Men sit on the elevated benches overlooking the Thames and smoke and stare; facing the river before they face the day.

Why does the city smell differently? I found a bit of research from the University of York which might provide part of an answer. They looked last year at the number of people accessing drug treatment services over the past ten years and found, not a rise in overall users, but a dramatic rise in the number of people over 40 smoking cannabis, nearly 120 per cent.

Now these are results I can believe in. It’s what I spy from my bike: not kids out for kicks, but ordinary grown-up men and women, people having a spliff at bus stops and on their way to work; smoking dope in the same routine way we once smoked fags.

On Sunday I saw a mother with a pushchair having a puff in the park. The dope smoke was curling gently around her small daughter’s hair, but no one gave the scene a second look.

Well, why would they? The police don’t care, and besides there’s been such a fuss made about the medicinal qualities of cannabis that it’s on the verge of seeming like an ethical lifestyle choice.

The Times recently interviewed a handful of aspiring marijuana magnates all longing for legalisation. One of them suggested that it might be used to help cure cancer.

Yes. But what if it’s also driving us mad?

Peter Hitchens wrote a letter to The Spectator last week, in which he mentioned a book by the former New York Times reporter Alex Berenson called Tell Your Children: The Truth About Marijuana, Mental Illness, and Violence. Hitchens wrote: ‘In Finland, Denmark and the US, recorded instances of mental illness have sharply increased as unfettered use of marijuana has become more widespread. In the US state of Washington, the first to legalise marijuana, the crimes of murder and aggravated assault have risen far faster than national averages.’

There’s been an argy-bargy about Berenson’s book, with some people arguing that these are correlations, not causes. But there’s nothing wrong with pointing out correlations — especially when the stakes are so high.

And the picture he paints of American cannabis use feels eerily familiar. American potheads are smoking drugs in a different way, he says. Look at it like this: ‘Only one in 15 drinkers consumed alcohol daily; about one in five marijuana users used cannabis that often.’ I’m betting the streets of New York smell the same as ours. And if cannabis is so benign, how come the grown-ups using it are increasingly begging for help? Those figures analysed by York University came from treatment centres, remember.

My theory — even less scientific than Berenson’s — is that marijuana affects different people differently; moreover that it’s very particularly the anxious types, most disposed to use it to take the edge off, who are in fact in most danger from it.

Didn’t we all know people for whom taking great handfuls of pills seemed no more significant than having a beer? They breezed through acid trips; whited out on cannabis; then came round beaming and ready for more. For others, like me, anything remotely hallucinogenic was a trip to Hell. ‘Just take half a tab,’ said my good friend back in 1997. ‘Feel the love.’ What I felt was such forceful gusts of raw terror that it took me half a decade to recover. ‘Oh,’ said the friend. ‘Try smoking grass. It’s calming.’ But all I felt was echoes of that same terrible fear. And I learnt to say, as I’ve heard others say: ‘I’m just not the sort of person who should do drugs.’

Come the future, perhaps we’ll be able to scan our genomes for dope sensitivity. Until then, aren’t we better off listening to Berenson than to some marijuana magnate?

Dope has good PR. The stoner on screen and in the public imagination is just a sleepy, hungry dude watching clips of dogs adopting ducklings on YouTube. The last thing you’d expect a pothead to be is angry. But a little--reported recent study from the University of Montreal paints a different picture. If you’re prone to psychosis, if you’re that way inclined, the drugs just make it worse.

Researchers studied more than a thousand patients released from a psychiatric hospital and discovered that the ones who smoked dope were more than twice as likely to be violent. In a way it’s no surprise. Neuro-imaging shows clearly that chronic weed smokers wither their prefrontal cortex, the part of the brain that inhibits impulsive behaviour.

According to one of the researchers, ‘an interesting feature of our results is that the association between persistent cannabis use and violence is stronger than that associated with alcohol or cocaine’. Interesting and profoundly depressing. Think of all the angry young men and women self-soothing with grass, not knowing that it’s dissolving the last shreds of the self-restraint they need so much. Think of their children.

And if the smell of the city seems sad, that’s why.