A common argument in favour of legalising cannabis is that alcohol is legal and is more harmful to people and society than cannabis, and therefore cannabis should be legal too.

This is a somewhat spurious argument along the lines of “Would you rather be eaten by a lion or a bear” or “Would you rather be run over by a truck or a bus”. Nevertheless, it provides a good opportunity to examine the differential harms posed by both substances.

A recent article by a Professor of Psychiatry and Addiction Medicine (NZ Herald, July 28) stated, “both cannabis and alcohol are known to cause psychotic conditions, but in both instances, these are rare events.”

This statement is true in one regard. Psychotic disorder related solely to alcohol is indeed a rare event. It is generally in the form of an uncommon condition called Alcoholic Hallucinosis, which occurs in older individuals who have been using alcohol heavily for many years. I have seen only one or two cases in my clinical career.

Individuals who are withdrawing from alcohol can suffer with hallucinations and acute paranoia as part of the withdrawal reaction but this is not a psychotic disorder. Cannabis, on the other hand, is strongly associated with psychotic symptoms and psychotic disorders such as schizophrenia.

In fact, cannabis use is now the most powerful single environmental risk factor for psychotic disorder.

Many well-designed studies have examined this association, and the majority have shown that cannabis is significantly related to psychotic disorder with the remainder showing a strong trend in that direction.

In the early 2000s, I was involved in a research team investigating the link between cannabis use in adolescence and the risk of psychotic disorder in adulthood. This research was carried out on a group of approximately 1000 young people from Dunedin, who had been followed up since the 1970s (and are still being followed up to this day).

We found that young people who started using cannabis before age 15 had had a four times increased risk of being diagnosed with schizophrenia or schizophreniform disorder by age 26. To put this in perspective, 10 per cent of the young people who had been using cannabis by age 15 developed a psychotic disorder in young adulthood compared with 3 per cent of the remainder of the group – a one in 10 chance is certainly not a rare occurrence.

The association between cannabis and psychosis appears to be getting even stronger in line with the increase in strength of cannabis (the THC content is now regularly over 20 per cent, whereas it was only about 1-2 per cent in the 1960s and 70s.)

Recent studies from Europe have examined the risks associated with high-potency cannabis (defined as greater than 10 per cent THC) and have found that daily users of high potency cannabis have a nine-fold increased risk of developing schizophrenia or another clinical psychotic disorder.

This “psychotogenic” effect is not confined to adolescent-onset cannabis use and appears more linked to frequency of use.

In addition, it is not confined to people with a history of psychotic disorder in their family (although they are at high risk and should avoid cannabis). It has been estimated that between one third to half of all the cases of psychotic illness in places like London or Amsterdam could be due to cannabis.

In my clinical experience, I have observed an increase in presentations of young men with psychosis related to cannabis over the past few years. The type of psychosis associated with cannabis is one with high levels of agitation, aggression and paranoia and can present a risk to family and others.

In a groundbreaking study in the UK, 16 healthy volunteers were given one dose of intravenous THC.

Remarkably, 70 per cent developed psychotic symptoms within 20 minutes and this demonstrates the powerful impact of the THC component of cannabis on psychosis.

Aside from its effect on psychosis, several large international studies have shown that cannabis use in youth has a more detrimental effect on cognition and later functioning in adulthood than alcohol use in youth.

A study from Christchurch showed that young cannabis users had greater rates of school dropout, unemployment and dependence on social welfare payments than their peers did even after taking account of alcohol use.

The Dunedin study showed that young cannabis users had lower incomes later in adulthood than their alcohol using peers. Heavy, early onset cannabis use has been associated with up to 8-point drop in IQ that appears to be irreversible.

No one would deny that alcohol is associated with a host of social problems and serious physical health problems.

However, I would argue that cannabis use is actually more damaging to the brain – especially the brains of young adults and teenagers, and their life prospects.

The failure of governments worldwide to control alcohol harms shows that once an addictive substance is legalised and freely available public health takes a second place to profit.

Once society normalises the widespread use of a drug it is almost impossible to undo that and put the genie back in the bottle.

• Mary Cannon is Professor of Psychiatric Epidemiology and Youth Mental Health, with the Royal College of Surgeons in Ireland.