A big stick is not the only way to fight cannabis use

But are the prime minister and others right when they say cannabis has been the cause of rises in mental illness in Australia?

While there was a marked increase in cannabis use among the Australian population from the 1970s to the end of the 1990s, there has been no change in the incidence of schizophrenia among the population during that time. Has it caused major increases in depression and anxiety? We simply don't know at this stage.

Is the prime minister also right to say that "re-criminalising" cannabis will result in decreased use?

The evidence, most recently reviewed by Associate Professor Simon Lenton, deputy director of the National Drug Research Institute in 2005, says No. Studies of 11 US states that decriminalised cannabis in the 1970s showed no increase in use, nor more favourable attitudes towards cannabis.

South Australia introduced the use of civil penalties, but the National Drug Strategy Household Surveys from 1985 to 1995 have shown that this approach has not resulted in increases in weekly use rates among young people any greater than in states that have not changed their laws.

So what approaches should we take to reduce harm?

Last year in the Victorian Premier's Drug Prevention Council, we undertook research among 13 to 29-year-olds, both users and non-users. It showed that we should use graphic imagery and realistic situations to illustrate the physical side effects of long-term and heavy marijuana use. These include depression and anxiety, as well as the social downsides, such as loss of friends and the effects on family.

And the research recommended against using the "just say No to cannabis" approaches, which stereotyped users with moralistic overtones.

It also advised against saying using marijuana isn't fun or doesn't have upsides (because it can be, and it does have upsides for many users), and said the messages shouldn't be delivered by medical professionals or government officials.

Our challenge is to ensure that the harm from drugs such as cannabis, just as with tobacco and alcohol, is reduced to the minimum possible. To do that we need broad prevention approaches focusing on the harms and links to mental illness.

These campaigns have to be credible, clever and appealing. And they have to be repeated year in year out to have any effect. A "just say No" scare campaign every five years isn't good enough.

These broad approaches have to be complemented by targeted interventions that focus on vulnerable children who are likely to drop out from school. We also need to ensure that people with a family or personal history of cannabis avoid use.

To be truly "tough on drugs" - that is, to be effective in reducing harm - a lot more must be spent on prevention and treatment, not just on law enforcement.