Drugs will accentuate mental illness if abused over a prolonged period of time. That is to say that a mental illness that is present in a person will be heightened by cannabis use. The equivalent of having a severe laceration on your forehead and thinking that repeatedly smashing a frying pan into it will make it better. It will not. Don't try this at home.
Scientific studies have located beneficial medicinal uses for cannabis. However, it should be pointed out that smoking cannabis, as opposed to eating it, is more likely to cause cancer. A bit like pointing out that putting an electrical appliance in the bath with you is more likely to cause electrocution. Guess what? Cannabis consumption alters your mental state; of course it does, that's the idea! Similarly, in a survey of people crossing the road, those with their feet tied together are more likely to be hit by a car.
It boils down to a matter of common sense. Quite simply, if you let a drug take over it will affect your personality. In the short term, having a night on cannabis leaves you waking up in the morning feeling fine. A night on alcohol leaves you waking up in the morning feeling like you have been kissing a donkey's arse and your brain feeling like it has been penetrated by a donkey's cock. Moderation is the key. People cannot function properly when abusing any drug on a regular basis and that is the simple truth.
Medical advisers for the government have ascertained that cannabis is not really that dangerous and that in countries where cannabis has been legalised there is a shift from which people drink less alcohol and their choice of recreational drug taking is cannabis.
So, it could be that the government's real fear is that if cannabis were to be legalised, people would drink less alcohol, thus reducing the millions of pounds that the government make from taxing alcohol. And also, people would grow their own cannabis to consume rendering the government unable to make millions from taxing cannabis, thus leaving the government massively out of pocket.
Therefore, the government invent stories of how cannabis is a dangerous drug, worse than alcohol and attempt to demonise cannabis by making such sweeping claims as that cannabis abuse in young people leaves them susceptible to mental illness and the destruction of brain cells.
It may be true that cannabis is not good for young people's developing mind, but it is no more dangerous than abusing alcohol in your teenage years.
The government's ultimate fear is that they can't work out how to make the maximum financial gain that cannabis consumption generates.