The Gateway theory

The issue of whether cannabis leads to the use of harder, more dangerous drugs has been a controversial subject for many years. One side of the argument states that if someone uses cannabis they will begin on a path to using harder drugs.

The other side of the argument says there is no evidence of cannabis causing the use of harder drugs, and that links between cannabis and other-drug use is due to societal and legal factors, as well as a person's predisposition to risky behaviour.

A study by Otago University researcher Professor David Fergusson, published in 2006, indicated that nearly 80 percent of the 1000 15 to 25 year-old people surveyed had tried cannabis, and over 40 percent had used other illicit drugs.

This study found a clear tendency for those using cannabis to have higher usage rates of other illicit drugs, but concluded that a number of reasons could be responsible.

• experience with cannabis may encourage experimentation with other drugs
• because cannabis is illegal, people who obtain cannabis come into contact with the illegal market and are therefore exposed to drug dealers
• cannabis might actually change the brain's chemistry to make young people more susceptible to trying other drugs.

In other research cited by the Health Select Committee's 2003 report into cannabis use in New Zealand, it was found that people who had used cannabis more than 50 times a year were 60 times more likely to try other illicit drugs than young people who had never tried cannabis.

However, again this study did not identify one singular causal link with the Committee report concluding: "These findings suggest that cannabis, when used frequently, may be a 'gateway' drug to other illicit drug use, although whether this is a result of contact with the illegal market or an effect of cannabis use is uncertain."