Frequent cannabis use can trigger depression, a study suggests.
Researchers have also found further evidence the drug can significantly increase the risk of schizophrenia.
The risks are outlined in three papers in the latest issue of the British Medical Journal.
Cannabis is not a risk-free drug. The public needs to understand the potential dangers of triggering mental illness.
Cliff Prior, Rethink
Researchers say their findings highlight the need for measures to reduce frequent and heavy use of cannabis.
The first paper, by doctors in Australia, found frequent cannabis use among teenage girls in particular can trigger depression.
Their seven-year study of 1,600 teenage girls found girls who used the drug everyday were five times more likely to become depressed and suffer from anxiety compared to those who did not use the drug.
Those who used the drug at least once every week were twice as likely to develop depression compared to non-users.
A second study, by doctors in Sweden, confirmed previous research suggesting that cannabis can increase the risk of developing depression.
Their study of more than 50,000 men found those who had smoked the drug in the late 1960s were 30% more likely to have developed schizophrenia
BBC NEWS | Health | Cannabis link to depression
The evidence is there no matter how much you object.