As we creep back into the open, as the Covid wards empty and the mental health clinics fill up, how are we going to tell what’s driven people crazy: lockdown, or what seems to have been a favourite lockdown hobby — smoking weed?

Last week Sadiq Khan, London’s goblin mayor, announced that if re-elected he’ll set up a commission to look into the case for decriminalising cannabis. It’s not in Khan’s gift to decriminalise anything — Downing Street has already issued a response which amounted to: ‘Decriminalise dope? You must be high.’ But Khan doesn’t care. This isn’t about the policy, it’s about the posturing. The race for City Hall is on, and Khan knows which way the wind blows on dope — more than 60 per cent of London residents say they’d back legalisation, and just 19 per cent are against it. If the smell on the streets is anything to go by, the remaining 21 per cent would be in favour too, if only they could speak.

There’s a decent case for legalisation — though decriminalisation seems like a strange halfway house. If cannabis can be regulated and legally sold, the drug-selling gangs might lose their grip, and London’s own Hunger Games, the annual summer knife-crime turf war, might finally end.

But making it easier to get stoned, removing the stigma, is not a risk-free business. There’s evidence that cannabis, in its 21st century incarnation, thick with the psychoactive ingredient THC, can send people clean round the twist. So my hope for Khan’s commission, after his inevitable re-election, is that it looks at the effect of decriminalisation on the most vulnerable: the mad, the anxious and the young.

Lockdown has been catastrophic for anyone mentally ill. If you’ve been out and about you’ll have seen the men and women rocking and muttering, and you might have noticed that they’re often self-medicating with dope.

There’s a schizophrenic man who paces up and down the pavement outside our local park, smoking weed and shouting at the air: ‘I’m going to fucking kill you all.’ I know he’s schizophrenic because on Friday he stopped in the middle of the road and yelled: ‘I’m schizophrenic. Help me.’ And I wish I’d had the bottle to say: stop smoking weed, it only makes it worse. Smoking, vaping or otherwise taking strong cannabis is a terrible idea for people with psychiatric disorders. But as studies increasingly show, it’s also a pretty unwise way for anyone to de-stress.

In March last year, as the Covid show began, theLancet Psychiatry released a paper with an alarming title: ‘Single dose of psychoactive component in cannabis could induce psychotic, depressive and anxiety symptoms in healthy people.’ It wasn’t a small study, it was a systematic review and meta analysis of 15 studies, and it concluded that even people with no history of mental illness can smoke themselves psychotic.

I can testify to the truth of this. I was at university in the late 1990s when strong, THC-thick skunk went mainstream. I still remember the surge of full-body terror, just a few minutes after smoking it; the uncontrollable, inexplicable fear. And once you’ve been that scared, it haunts you.

Then there’s the young. You might say all our pleasures have downsides; alcohol and tobacco kill too, let adults pick their poison. But in places where dope’s been legalised, even with an age limit, inevitably more teenagers have a try, and it looks like it affects their brains irreversibly. The New York Times ran a depressing piece a few weeks ago reporting that cannabis is highly addictive for adolescents. Early use seems to prime young, plastic brains to crave more, and three years after first trying the drug, 20 per cent of adolescents had become dependent on it. Dependency means trouble concentrating, learning and reasoning. Another study in JAMA Psychiatry of more than 23,000 people found that the risk of depression among young adults who regularly smoked cannabis was more than a third higher than normal, and their risk of attempting suicide more than trebled.

We all know it’s the least fortunate who’ve suffered most this past year. I would just like Mayor Khan’s commission to be sure, as they consider the evidence, that sanctioning weed won’t simply clobber the same poor people again.

Some time around 2006 my then flatmate, a filmmaker, had a good idea: why not make a programme of reverse anthropology? Instead of going to the jungle with some square-jawed presenter to marvel at the natives, he decided to bring the natives here, to England, to see what they made of us.

The islanders who arrived one drizzly day were from Tanna, in the South Pacific, and their particular interest in coming here was to meet god, aka HRH Prince Philip.

Before they arrived, I felt a certain amount of patronising anxiety. They believed that Prince Philip had emerged from a volcano on Tanna. Would I be able to keep a straight face? My fears were unfounded. Everyone who met them agreed: Chief Yapa, Joel, Posen, Albi and J.J. were significantly more enlightened than us: joyful and direct.

When word came from Windsor that Prince Philip would meet his followers, my anxiety transferred itself: would he disappoint them? How could he not? Never meet your heroes, they say, let alone your god.

When the great day dawned, the islanders were ready. Prince Philip had decided cameras were not to be allowed, so the final episode of my friend’s series Meet the Natives showed them walking through the royal door, as if into a church sanctuary, all five in their Sunday best. Chief Yapa had brought a question with him from Tanna that he said Prince Philip would be sure to understand: ‘When will the pawpaw tree be ripe?’

As we waited for them to emerge again, I imagined Prince Philip making some awful joke; the disappointment in Chief Yapa’s eyes. But when Chief Yapa came out, he was grinning. They’d had a long, respectful conversation, he said. Prince Philip had thought about their question seriously and replied in a very satisfactory way. As my filmmaker friend remembers it, the Prince said: ‘The pawpaw tree is not yet ripe.’ The BBC reports that he said: ‘When it turns warm, I will send a message.’ Whichever it was, god played a blinder.