Cannabis: the scientific evidence against it

by NAOMI COLEMAN and ROSALIND RYAN,
The news that cannabis could soon be available on the NHS and has been reclassified as a 'Class C' drug has left many people confused about the health risks and benefits of this drug.
To find out the truth about cannabis, here, we look at the scientific evidence against it

THE EVIDENCE AGAINST CANNABIS
• Teenagers who smoke cannabis are more likely to develop mental illness
Recent studies suggest cannabis users are at least six times more likely than non-users to develop schizophrenia
A study, published in the British Medical Journal, found that one in ten people who used cannabis before the age of 15 developed schizophrenia by the age of 26. This compared to three per cent of those teenagers who did not use the drug.
Another study carried out by experts at Australia's Institute of Victoria showed that teenage girls who used cannabis were more likely to suffer depression and anxiety than other adolescents. Daily use of cannabis was linked to a five-fold increase in risk, while those who took the drug weekly doubled their chances of developing the illness.

• Smoking cannabis is more harmful to your lungs than cigarettes
Just three cannabis 'joints' a day can cause the same amount of damage to lungs as an entire packet of 20 cigarettes, according to the British Lung Foundation. The foundation found that tar from cannabis joints contain 50 per cent more cancer-causing toxins than cigarettes from tobacco alone.
Worse still, users tend to inhale four times more deeply than with tobacco leading to higher levels of respiratory conditions such as bronchitis. Other studies show that benzyprene, found in the tar of cannabis joints, can change the makeup of one of the genes that suppresses tumours and could therefore make cancer more likely for people who smoke joints.

• Marijuana is as addictive as other drugs.
Drugs such as heroin, cocaine and alcohol trigger a surge of chemicals in a particular area of the brain known as the 'pleasure centre'. Experts say this is the hallmark of an addictive substance. Animal experiments have now found that cannabis also produces a surge of chemicals in the same area, leading to claims that the drug must be more addictive than previously thought. However, critics say there are many non-addictive drugs that also stimulate this chemical surge.

• Cannabis can affect users' memory and concentration.
A recent study carried out by America's John's Hopkins University found regular users of cannabis - those who smoked up to 12 joints a day, even after abstaining for a month - performed worse in mental agility tests than those who only smoked once or twice a day.
The researchers concluded that cannabis use can cause similar changes to those after a brain injury. Another study by psychologists at America's Virginia Polytechnic Institute and State University found that long-term marijuana smokers - those who had smoked for an average of 24 years - had shorter attention and memory spans than short term users; those who had smoked for ten years or less.