Cannabis use 'third highest in world'

New Zealanders are among the highest users of cannabis in the world, according to UN figures quoted in a medical magazine.

They rate behind the United States and Australia but ahead of European nations.

An estimated 166 million people worldwide have either tried using cannabis or are active users of the drug despite scientific research showing its adverse effects on health, two researchers in Australia said.

The figure, taken from the United Nations Office on Drugs and Crime (UNODC), means one in every 25 people between the ages 15 and 64 in 2006 had had some experience with the drug, the researchers wrote in a paper published in The Lancet.

The paper was written by Professor Wayne Hall at the School of Population Health in the University of Queensland in Brisbane and Professor Louisa Degenhardt at the National Drug and Alcohol Research Centre in the University of New South Wales in Sydney.

It contains findings of the UNODC and a review of medical literature published over the past 10 years on the adverse effects of cannabis on human health.

Cannabis use was highest in the United States, Australia and New Zealand, followed by Europe. Because of their large populations, 31, 25 and 24 percent of the world's cannabis users were from Asia, Africa, and the Americas respectively, with Europe accounting for 18 percent and Oceania 2 percent.

In New Zealand and Australia more than 8 percent of the population aged 15-64 used the drug. But use was declining in both places, the researchers said, adding that similar trends were seen in Western Europe.

In contrast, cannabis use was increasing in some low- and middle-income countries, such as those in Latin America and several countries in Africa.

The paper cited North American research showing that 10 percent of people who had had contact with cannabis became daily users, while 20 to 30 percent became weekly users.

In the United States, Australia, and New Zealand, regular cannabis users were much more likely to use other illicit drugs later on, including heroin and cocaine, they added.

Cannabis use typically began in teenage years, peaking in the early- and mid-20s, before declining as young people entered full-time employment, got married and had children.

Use of cannabis in pregnancy could reduce birthweight, but did not appear to cause birth defects, they added. Regular cannabis smokers reported more symptoms of chronic bronchitis, such as wheezing, sputum and chronic coughing, than non-smokers.

Deficits in verbal learning, memory, and attention were most consistently reported in heavy cannabis users, but these were related to duration and frequency of use.

Cannabis also appeared to have an effect on the mental health of users, the researchers wrote, citing studies which suggested that the risk of schizophrenia more than doubled in those who tried cannabis by age 18.

"The most probable adverse effects (of cannabis) include a dependence syndrome, increased risk of motor vehicle crashes, impaired respiratory function, cardiovascular disease, and adverse effects of regular use on adolescent psychosocial development and mental health," they concluded.