Day in and day out, I see the tragic human fallout from our lackadaisical attitude towards cannabis. I work in a specialist service for people with emerging severe mental illness and most of my patients have experienced psychosis.

The vast majority of them have used cannabis, and their lives have been shattered by this dreadful drug. Around one third of psychosis cases in the capital are the result of cannabis, according to research by Sir Robin Murray, a professor of Psychiatric Research at King’s College London.

These studies join the hundreds of others showing that, far from being the benign substance campaigners would have us believe, cannabis is a dangerous intoxicant that has a profound effect on the structure and function of the brain.

Several times a day, every day that I’m at the hospital, I meet patients whose minds have been broken by cannabis. I implore them, for the sake of their own sanity, never to smoke it again, but it is an uphill battle because they just do not seem to understand its impact on their mental health.

And who can blame them? Our political class long ago gave up trying to stop people using it. The police couldn’t care less. People are now emboldened to smoke it openly in public. Walk down the street in central London and you are constantly confronted by the sickly whiff of weed.

I hate this drug because I see the lives that it destroys. This is why it makes me so angry to hear the likes of Sadiq Khan, the Mayor of London, last week recommend that it be partially decriminalised. Has he lost his mind? Does he not see what this will do? He is a classic example of the ostrich class, as my colleague Peter Hitchens described him and his ilk in this newspaper last week.

Sadiq Khan, the Mayor of London, has recommend the partial decriminalisation of cannabis

He is so detached from the reality of what this drug does to people and communities.

Khan banned images of scantily clad women on the Tube in case it triggered eating disorders, yet advocates allowing people to use cannabis. It makes no sense.

I spent ten years working with eating disorder patients and I can assure you that not one ever told me they ‘caught’ anorexia from a poster on the Tube. Yet I’ve seen countless people whose lives have been ruined by cannabis.

Data from our neighbours in Europe offers an insight into the potential pitfalls of Khan’s proposal, with Portugal seeing a huge surge in cannabis-induced psychosis after it decriminalised the drug in 2001. The more people who use it, the more lives will be destroyed.

It has now been proved beyond doubt that cannabis is associated with depression, anxiety, psychosis and avolition (poor motivation).

It seems to me a bitter irony that, just as we are becoming more aware of mental illness, the use of a drug directly responsible for blighting the mental health of so many people is being allowed to become so widespread.

It has been proved beyond doubt that cannabis is associated with depression, anxiety, psychosis and avolition

Furthermore, the idea of decriminalisation is nonsensical. Surely, there should only be two options: enforce the law or legalise cannabis. If we are going to accept that we have lost the war on this drug then, rather than turn a blind eye to its use, we must make it legal, regulate it and control it.

Khan’s policy is the worst possible option. Decriminalising cannabis means it’s still illegal, but there are no consequences for using it.

This risks sending a dangerous signal to criminals, and to our young people – that the law doesn’t matter and is there to be broken.

Teens will surely then assume that a lax attitude means that, really, it’s no big deal.

And, given that drug dealers are unlikely to check a customer’s date of birth – unlike with the purchase of alcohol or tobacco in shops – in a decriminalised system, cannabis effectively has no age restrictions.

Think about it – it means there would be more control over who can buy the cigarette papers used to make a joint than there is over the drug that goes in it.

Of course, it also means that criminals will continue to make money. As studies have shown, crime has risen, not fallen, in countries that have decriminalised cannabis.

Still think Khan might be on to something? What about the fact that cannabis farms are linked with modern slavery, people trafficking, prostitution, guns and violent crime?

It is a world of utter misery. It destroys communities.

As much as the thought sends shivers down my spine, I’ve come around to the idea that there might be an argument for legalising cannabis.

At least then it can be taxed, regulated and produced without slave labour. The type of cannabis being sold could be monitored, with the level of potency kept in check, regulations placed on its sale and controls introduced around who is selling it.

The taxes raised could be used to fund specialist services like the one I work for to help mop up those whose mental health will continue to suffer due to the drug, whether or not it is legal.

But, no, Khan plays to the spliff-smoking crowd and calls for decriminalisation – an approach that will ruin lives.

I applaud Kara's cancer honesty

Kara Tointon has spoken out about her double mastectomy after finding out she was at high risk of breast and ovarian cancer

Former EastEnders actress Kara Tointon has undergone a double mastectomy following a gene test.

The 41-year-old revealed that tests showed she carried a mutated BRCA gene – which can put her at very high risk of breast and ovarian cancer.

She was tested in 2018 when her late mother was undergoing treatment for ovarian cancer.

Last week she posted about her mastectomies on social media.

In general, I’m not a fan of celebrities revealing all about their health woes because it tends to encourage health anxiety in people who are perfectly well. But, in this case, I applaud her. It cannot have been easy for her to talk about, but many women simply don’t know about this gene mutation and the risks attached to it.

It’s not until they have symptoms themselves and a surgeon asks about a family history that it becomes apparent they are carriers. That’s a tragedy because, if they had been tested before, then they might have avoided cancer altogether.

I loved shadow justice secretary Robert Jenrick’s crusade against fare dodgers. He confronted people on the Tube who hadn’t bought a ticket. I recently challenged someone who tailgated me through a barrier and was met with abuse. Why not give Tube workers financial incentives to stop it? It would help oppose an underclass which believes it can behave however it likes.

Dr Max prescribes... Free online exercise class

I’m determined to do what I can to help tackle the obesity epidemic and so have started a free online exercise class (getslimmr.co.uk/exercise) for those middle-aged or older who would like to gently improve their physical activity from the comfort of their own home. It’s on a Wednesday at 7pm and run by personal trainer Ben Hodson.