Smoking dope is now as common as smoking tobacco among teens and young adults. Use has grown tremendously in the past 20 years.
However, the study confirmed that although it is as addictive as heroin or alcohol, it is impossible to fatally overdose from it.
An WHO expert adviser on addiction reviewed cannabis research since 1993 and set out the effects of cannabis use on mental and physical health. He found that people dependent on cannabis could not work out how to give it up.
Around one in 10 of all cannabis users and one in six among those who start in adolescence do become addicted.
A spokesman for the charity Rethink Mental Illness stated that cannabis is wrongly seen as a safe drug. There is a clear link with psychosis and schizophrenia, especially for teenagers.
Regular cannabis users double their risks of experiencing psychotic symptoms and disorders, especially if they have a personal or family history of psychiatric episodes, and if they start using cannabis in their mid-teens.
Teenagers who smoke dope regularly are at double the risk of being diagnosed with schizophrenia or reporting psychotic symptoms in adulthood.
Teenage cannabis smokers do worse at school achieving poorer exam result.
The study suggests teenagers will smoke into the adulthood impairing intellect, but the mechanism and reversibility of the impairment is unclear.
Regular cannabis smokers have a higher risk of developing chronic bronchitis and if they continue into middle age probably increases the risk of myocardial infarction, a heart problem.
Driving while cannabis-intoxicated doubles the risk of a car crash which increases if combined with alcohol.
Smoking whilst pregnant harms your baby by stunting its growth.
The report is published online today in the scientific journal Addiction.
Okay let's look at the facts about the plant.
Cannabis is a genus of flowering plants that includes three different species, Cannabis sativa, Cannabis indica and Cannabis ruderalis, all indigenous to Central and South Asia.
BBC: Cannabis comes in many different forms. Hash or solid is cannabis resin, which is a dark or light brown solid. Marijuana or grass is the dried leaves and flowers of the plant. Some varieties of the Cannabis plant are more potent, and these are often referred to as Skunk.
More recently there has been an increase in strong strains of weed. This includes Skunk and some 'homegrown' varieties. They can be 2 - 3 times stronger than normal weed and often have a more potent smell.
Cannabis can be smoked with or without tobacco. It is either rolled up in cigarette papers as a joint, or smoked from a pipe, or bong (water pipe).
Some people bake hash in cakes, drink it in tea or just eat it on its own.
Regular cannabis use is known to be associated with an increase in the risk of later developing psychotic illnesses including schizophrenia; and if you have a family background of mental illness, you may also have an increased risk.
Some people use cannabis to relieve the symptoms of multiple sclerosis or chronic pain.
The next few years caused great trauma. Instead of the expected good reception, other students picked on him, mainly because he refused to fight back. Cut to the subject in hand: he eventually defended himself, conquered the aggressors, and secretly started smoking cannabis. As his sisters gained their teenage status, they too indulged on secret trysts at the beach.
That's when I learned about peer group pressure overcoming the example and teaching from parents. That's when my childrens' beautiful spirits were tainted. It ended in sorrow. The two oldest children are dead and the youngest still insists cannabis is a wondrous natural plant. Their father does have mental health problems and severe depression, which may be the major influence.
Do you have a story to tell about the effects of cannabis—be it good or bad?