Anti-drugs campaigns can increase the risk of drug use, a report to Home Secretary Theresa May has found.
Some preventative work, including drug education in schools and mass-media publicity campaigns, has "little impact on preventing substance abuse" when used in isolation, according to scientists.
The Advisory Council on the Misuse of Drugs (ACMD) report also found that " international evidence suggests many popular types of prevention activity are ineffective at changing behaviour, and a small number may even increase the risks for drug use".
It said there is " little clear evidence of what works in drug prevention" though targeted "interventions" can help individuals at a high risk of harm.
Professor Les Iversen, who chairs the ACMD, said: " This research demonstrates that there is more to be done in order to understand the complex network of substance abuse prevention programmes operating in the United Kingdom.
" Better analysis of the merits of these programmes will help policy-makers and commissioners to make best use of limited financial resources, with the ultimate beneficiaries being the service users themselves."
It comes after Liberal Democrats unveiled plans for a radical shake-up that would make c annabis available on the NHS and see people found with small quantities of drugs spared the threat of jail or a criminal record.
Under the proposals, doctors would be able to prescribe cannabis for medicinal reasons and a review would be set up to consider the effectiveness of experiments in legalising the drug in parts of the United States and Uruguay.
Deputy Prime Minister Nick Clegg said the Lib Dems would commit to adopting the approach used in Portugal, where people arrested for possession of drugs for personal use are diverted to treatment, education or face civil penalties instead of facing criminal sanctions.
He said that people who used drugs should be seen as the "victims" of the dealers who sold illegal substances.
"The victims need to be treated with compassion, with treatment and all the best expertise that clinical excellence can deliver to wean them off their habit," he said.
A Home Office spokesman said: "Preventing and reducing drug misuse is a key part of our drug strategy and there are positive signs our approach is working.
"There has been a long term downward trend in drug use over the last decade, the number of heroin and crack cocaine users in England continues to decline, and more people are recovering from their dependency now than in 2009-10.
"Our evidence-based approach recognises the importance of having a range of prevention initiatives to give individuals the confidence to resist drugs.
"We welcome the ACMD's advice on this important issue."