The British Medical Association has launched a last minute attack on the government's decision to downgrade the criminal status of cannabis.
Cannabis use will not lead to arrest in many cases
Cannabis is due to be reclassified from a class B to a class C drug next week.
Doctors' leaders said they were "extremely concerned" the move would mislead the public into thinking the drug was safe to use.
Tony Blair insisted possession of cannabis remained illegal and
users could be arrested.
Doctors' leaders warned cannabis has been linked to greater risk of heart disease, lung cancer, bronchitis and emphysema.
Dr Peter Maguire, deputy chairman of the BMA's board of science, welcomed a government campaign to highlight the drug's dangers.
"The BMA is extremely concerned that the public might think that reclassification equals 'safe'. It does not.
"We are very worried about the negative health effects of smoking cannabis and want the government to fund more research on this issue."
The mental health charity Rethink is also calling for clear health warnings to be issued over the increased risk of developing schizophrenia, and other forms of psychosis, from cannabis use.
Its chief executive Cliff Prior said: "There is a strongly-held view that cannabis is risk-free, reflected in the astonishingly high rates of use amongst young people as the street drug of choice.
He said cannabis makes the schizophrenia symptoms far worse in people who have the illness.
"Now there is a rapidly growing body of evidence showing that cannabis can trigger schizophrenia in people already at risk."
When downgraded possession of cannabis will no longer be treated as an arrestable offence in most cases.
Police will retain the power to arrest users in certain aggravated situations, such as when the drug is smoked outside schools but officers will not target home use.
Some drugs campaigners have criticised the move, saying it will encourage more youngsters to experiment.
But Mr Blair played down suggestions that the new law would send out "mixed messages" to young people.
He told the Commons on Wednesday the change in the law would allow the police to focus on tackling hard drugs such as heroin and crack cocaine.
About 80,000 adults a year are arrested and fined for cannabis possession.
Research has found the drug is far more harmful to health than smoking
ordinary cigarettes, with more tar and harmful chemicals taken into the body.
Around 120,000 people die every year in the UK from smoking cigarettes.
A £1m government advertising campaign is to underline the message that cannabis will remain illegal.
Marjorie Wallace, of the mental health charity SANE, said there was a large body of evidence showing how dangerous cannabis can be for those vulnerable to psychotic illness.
"We fear that the government education campaign will be too late, and that no amount of information will be able to counter the widespread acceptance of the harmless nature of what can be for those at risk a lethal and life-eroding drug," she said.