Cannabis can be as hard to give up as heroin, according to one of the largest ever studies into its effects.

Depression, anxiety and insomnia are a massive struggle for some heavy, dependent users, addiction expert Professor Wayne Hall said in the study.

Even after treatment, less than half of those people can stay off the drug for six months.

One in six teenagers who use regularly cannabis will also become dependent and are twice as likely to quit school, the World Health Organisation (WHO) adviser said.

Among adults, one in 10 become dependent and are more likely to try harder drugs.

Cannabis was a Class C drug in Britain from 2004 to 2009, but was reclassified as a more harmful Class B substance five years ago.

Pro-cannabis campaigners argue it should be moved back to Class C, or even decriminalised and sold in regulated shops - as has happened in many American states, such as Colorado and Washington.

But the findings of the 20-year review appear to support the status quo.

It also found the risk of developing psychotic disorders, including schizophrenia - long associated with the drug - doubles with regular, long-term use.

And driving after smoking a joint doubles the chance of a crash.

Professor Hall, a substance abuse expert at King's College London and the University of Queensland, looked at scientific evidence between 1993 and 2013.

"The perception that cannabis is a safe drug is a mistaken reaction to a past history of exaggeration of its health risks," Hall told Live Science.

But he also found the chance of taking a fatal dose of the drug was "extremely small" and found no recorded cases.

Professor Hall also admitted cannabis was "not as harmful as other illicit drugs such as amphetamine, cocaine and heroin, with which it is classified under the law in many countries".