"Oooh, she experimented with cannabis! String 'er up!" That was the glib response on an internet forum to a mother concerned about her teenage daughter smoking cannabis. The same user also helpfully suggested the worried mum needed to "chillax".

A random example from an online discussion perhaps, but it's indicative of a common view that cannabis is a harmless rite of passage. In reality, it can be much more serious.

Other examples of this laid back view about a potentially very harmful drug are rife. A colleague reports that one of her friends had been smoking the occasional joint to ease the pain of chemotherapy. On the wonderful news that she had the all-clear from breast cancer, she handed over the leftovers to her (extremely grateful) teenagers.

In an article headed 'Mum swaps Merlot for marijuana' £ in the Times earlier this year, the writer Lucy Cavendish described how it felt "terribly communal" to get together with friends to share joints.

Her rediscovery of the drug after 20 years is not uncommon. The Independent Drug Monitoring Unit figures suggest that 2.6 million adults use cannabis, with its use among the over-30s gradually increasing since 2004.

It's certainly true that smoking cannabis is far from rare. What is alarming however, is the view that doing so is pretty much harmless and that it's inevitable that most teenagers will dabble with it. The argument goes that it's part of growing up, it's not a big deal - even President Obama 'inhaled frequently' in his younger days.

Questioning this received wisdom is likely to provoke rolled eyes and sighs. To an extent, I understand why. For a start, I've inhaled too, albeit around 20 times in 20 years and not at all for 10. Most of the people I know have done too. And yes, there are much more dangerous drugs around, binge drinking is just as, if not more, harmful overall.

What is worrying, however, is the apparent ignorance which exists about the link between cannabis and severe mental illnesses such schizophrenia.

I was pretty ignorant too, until I joined Rethink Mental Illness just over a year ago. Since then, I've been taken aback by the number of people I've come into contact with who have schizophrenia and were heavy cannabis users in their teens and early 20s. It's a story we come across all too often. I met a parent who told me they knew their teenager smoked 'a bit of grass', but didn't think too much of it until he started having delusions about the devil.

Another told me about his desperation, trying to protect his child from the forces "destroying his beautiful mind".

Let's be clear, Rethink Mental Illness does not believe that every teenager who has the odd joint is going to wake up the next day with a full-blown mental illness. But what we are saying is that cannabis is not the 'safe' drug many believe it to be, and that we urgently need to get real about the potential risks.

Cannabis is a bit like nuts. I can eat nuts without fear, but my son had such a severe reaction to eating brazil nuts that we now have to carry adrenalin with us at all times. Cannabis is the same, some people react very badly to it, some hardly at all.

Studies suggest that cannabis smokers with a genetic vulnerability to psychosis are more likely to develop it, and at an earlier age. One study found that people who had used the drug before the age of 15 could quadruple their risk of experiencing psychosis. Given that, one has to question whether it is a risk worth taking. It's certainly important to challenge lazy thinking about the dangers.

One of the problems is that the cannabis many parents remember from their youth is a completely different substance to the one around today. Levels of THC, the dominant psycho-active agent in cannabis has been increasing year-on-year, resulting in a much more potent drug.

Despite this, Rethink Mental Illness didn't support the reclassification of cannabis from class C to B in 2009. That's because we don't believe that fiddling around with categories is the most effective way to tackle the problem: indeed, our own polling has shown only 3% of people would be motivated to quit cannabis because of stricter laws.

The critical issue is around education. The view that smoking cannabis is nothing to get worked up about needs to be challenged more effectively. Instead of classifying and re-classifying, government time and money would be much better spent on educating young people who smoke cannabis about the very real game of Russian roulette they're playing with their mental health.

Follow Mark Davies on Twitter: www.twitter.com/markdavies67