Tory leader Michael Howard has pledged to reverse the relaxation of cannabis laws as a new campaign is launched to remind people of the changes.
Doctors fear reclassification might make cannabis seem safe
Cannabis will be reclassified from class B to a class C next week despite doctors saying the drug is unsafe.
Mr Howard said the government's policy gave a "confused and muddled signal".
But Home Secretary David Blunkett defended the move, saying it would make drug laws credible and focus police efforts on the most dangerous drugs.
Mr Howard told BBC News reclassifying cannabis was a mistake.
"It's sending out a confused and muddled signal," he said.
"Indeed, the government is having to spend £1m of taxpayers money on an advertising campaign to try to clear up the confusion."
Mr Howard pointed to evidence the drug could cause serious mental health problems.
There was a respectable case for legalising cannabis, he argued, but not for the government's "muddle in the middle".
Mr Howard refused to reveal whether he had ever taken cannabis, arguing it would only prompt journalists to ask his shadow cabinet.
When former shadow home secretary Ann Widdecombe proposed a tougher line on cannabis in 2000, eight of the then shadow cabinet admitted using the drug.
But Sir Michael Rawlins, chairman of the Advisory Council on the Misuse of Drugs, said it would be "logically stupid" to regard cannabis in the same class as amphetamines.
"It actually means that one joint in the pocket can send you to prison for five years," he told BBC Radio 4's Today programme.
The Tory statement comes as the government seeks to clarify the new position, reminding users that the drug remains illegal.
The £1m campaign from the government's drugs information service, Frank, warns young people that possession is still a criminal offence.
Radio adverts will be played on 48 English national and regional commercial radio stations.
In Wales, Scotland and Northern Ireland, adverts and leaflets are being tailored to fit existing campaigns.
Two-and-a-half million leaflets outlining the latest drugs laws will be distributed.
Blunkett says current drugs laws have sent a confused message
Information packs promoting the message will be sent to all schools drug advisors, drug action teams, drug charities, health organisations and student unions.
Mr Blunkett told Today he wanted a "transparent, non-variable, understandable policy across the country".
Politicians had to take responsibility on "whether the police go for Class A dangerous drug pushers and users that kill or whether we go for small possession for cannabis".
He said some families of drug users were also saying that it was important to make clear the distinction between cannabis and harder drugs.
These families had told him: "If you confuse our children by saying pretty much that cannabis is the same as crack or heroin, when they take cannabis and find it is not, they do not believe the message when they go on to heroin or crack."
People found with small amounts of cannabis would be warned and could be arrested if they persistently offended, added Mr Blunkett.
Downing Street said it was not arguing cannabis was safe but it was less harmful than other class B drugs.
Independent charity Drugscope supports the reclassification of cannabis on 29 January and the adverts.
But it says some people may have been led to believe that the drug was being legalised.
Dr Peter Maguire, from the British Medical Association, said he would welcome efforts to highlight the drug's dangers.
But he added: "The BMA is extremely concerned that the public might think that reclassification equals 'safe'. It does not."