Gordon Brown publicly backed the decision to sack the Government's chief drugs adviser today.
The Prime Minister said Professor David Nutt's comments gave the impression the Government was sending "mixed messages" about drugs.
He said the Government could not risk sending out the message that it was acceptable for young people to experiment with cannabis, as this could lead to more becoming addicted to hard drugs.
He told an audience of police, council workers and members of the public in Shoreditch, east London, today: "Advisers advise and ministers have to make decisions."
He said: "I think Alan Johnson made the right decision because we cannot send mixed messages.
"Scientific advice is very important and we value it. You can see that with swine flu, with climate change and with all sorts of environmental problems.
"But advisers advise and ministers have to make decisions.
"In the interests of the public we have to show we are tough on drug dealing and the problems that drugs are causing in our communities."
He added: "We cannot send out a message to young people that it's OK to experiment with drugs and to move on to hard drugs. We have to send out a message to young people that it's simply not acceptable."
Mr Brown said young people needed to be protected from dangerous new strains of "adulterated" cannabis.
He went on: "I think the issue here is we did have advice that we should not reclassify cannabis. We did not accept that.
"We have to take a broader view in the round that was more than just the scientific advice. It's about the effects on young people that drugs are harmful and not acceptable."
He said Prof Nutt's "consistent disagreement" of opinion undermined the Government's message on drugs.
Prof Nutt was sacked by Mr Johnson on Friday, sparking a row between ministers and the scientific community.Reuse content