The minister with responsibility for drug policy, Aodhan O Riordain, said last week he wanted a "conversation" about decriminalising certain drugs. This was widely taken as referring to cannabis. He said full legalisation was not on the cards. So, what's the difference?

Legalisation means that any and all legal barriers to purchasing cannabis would be removed with regulation in a similar manner to those governing alcohol or cigarettes. In addition there would be no criminal implications for those who sell or buy it.

On the other hand, decriminalisation would lessen charges against casual users, meaning that anyone caught in possession of small amounts of cannabis would be subject to some penalty but would not face criminal charges in court.

Several states have recently decriminalised or fully legalised cannabis in the US. Cannabis use has been tolerated and decriminalised in Holland since the 1970s.

Coffee shops - specialising in cannabis, not coffee - mushroomed and were seen as safe places for those 18 years and over to relax and have a puff.

In the last number of years the Dutch have been examining the impact of this change, as concerns grew about drug tourism and the increasingly public use of the drug. As the concentration of tetrahydrocannabinol (THC), the main psychoactive substance in cannabis, increased over the years to a new norm of 16-18pc, worries about mental health and crime associated with its sale also rose.

The Garretsen Commission, established by the Dutch Government, recommended that cannabis with a concentration of THC higher than 15pc be made illegal and that tourists be banned from coffee shops.

The recommendations of this commission have now taken effect.

In Britain, under pressure from celebrities and from the British Medical Association, which claimed that cannabis had useful medical properties, the government decided to allow it for personal use in 2004.

By 2007 scientific evidence was emerging that cannabis was not as benign as had been thought and the Independent newspaper, which previously campaigned for its decriminalisation, now reversed its position with the front page headline 'Cannabis: An Apology'.

It continued 'If only we had known then what we can reveal today'. In January 2009, cannabis was upgraded again to a Class B drug in Britain.

The issue of decriminalisation is of crucial interest to psychiatrists. The key questions for us are whether cannabis decriminalisation will increase the usage of cannabis in the population as a whole and if this will increase the rate of mental illness.

The definitive study by Professor Wayne Hall, drugs advisor to the World Health Organisation, was published in the journal Addiction in October 2014. It found that one in six teenagers who regularly smoke cannabis become dependent on it; it doubles the risk of developing schizophrenia; regular use in the teenager impairs intellectual development; those who use it are more likely to use harder drugs such as heroin or cocaine, and smoking while pregnant reduces the weight of the baby.

Another review of the scientific research by Professor Nora Volkow, director of the US National Institute on Drug Abuse, and published in the June 2014 New England Journal of Medicine, reached broadly similar conclusions.

In her study 9pc of those who experiment with cannabis will become addicted, while withdrawal symptoms make cessation difficult and contribute to relapse.

Those of us working in the emergency departments of hospitals deal with the effects of cannabis every day. Brief psychotic reactions are common and while comfort might be taken from the word "brief", they can be terrifying for those who witness it and also for the person themselves.

These episodes may augur the onset of a more prolonged psychosis, auguring the transformation into schizophrenia. The effects on motivation and interest, while less dramatic than the experience of hearing voices and feeling persecuted, are well documented, destroying ambition and enthusiasm for life.

Those who already have mental illness fare even worse. Their experience relapses in their illnesses that impair their day-to-day living.

Decriminalisation gives everybody permission to use cannabis but unfortunately there will be casualties.

In an environment that says small amounts is okay, a vulnerable person is likely to be tempted. Just as they were when head-shop drugs were deemed acceptable, those with pre-existing mental illness, or with none, presented in their droves to our emergency departments, deluded, hallucinating and terrified.

The cannabis campaigners ignore these disturbing realities. But the truth is far less pretty than a relaxed young man having a "roll your own" in a coffee shop.

Health & Living