Mental health and drug charities have welcomed moves to reassess the class C rating of cannabis.
DrugScope said it was right cannabis be closely monitored
The drug was downgraded from class B in 2004, but Home Secretary Charles Clarke has ordered a review after new studies linking its use to mental illness.
Charity Sane said cannabis use "could trigger a journey of life-long disintegration" and it was "relieved".
DrugScope also supported the move, but warned a review must be scientifically based and not politically motivated.
A recent study by New Zealand scientists suggested smoking cannabis virtually doubled the risk of developing mental illnesses such as schizophrenia.
Paul Corry, of charity Rethink Severe Mental Illness, said Mr Clarke's order for a reassessment was a "victory" for those with mental illness.
"At last the government has woken up to the risk they have been running of a drug induced mental health crisis," he said.
"There is mounting evidence that cannabis dramatically increases the risk of developing schizophrenia in people where there is a family history of the illness, and significantly increases the risk even where there is no family history."
Mr Corry called on the Advisory Council on the Misuse of Drugs - which recommended cannabis be downgraded and has been asked to review its position by Mr Clarke - not to rest on its "flawed" previous advice but to listen to affected individuals and families.
He added that the Department of Health should launch a campaign to show that cannabis was not "a risk free drug".
"We want people to have the clearest possible understanding of the link between long-term and early age use of cannabis and schizophrenia," he said.
The head of mental health charity Sane, Marjorie Wallace, said the organisation had campaigned for over 18 years to highlight the dangers of cannabis.
"Far from being a relatively harmless recreational drug, for vulnerable teenagers the innocent spliff, or chilling out, could trigger a journey of life-long disintegration," she said.
The chief executive of drugs charity DrugScope, Martin Barnes, said: "DrugScope supported the reclassification at the time but if new evidence emerges to show that cannabis is more harmful than thought, this needs to be considered.
"It is right that the classification of cannabis, as with all drugs, is closely monitored on an ongoing basis.
"But we must ensure that such monitoring takes place on a rigorously scientific basis and is not motivated by political factors."