The UK drugs market is proving "extremely resilient" despite the large sums of money spent tackling the problem, according to a new report.
The UK Drug Policy Commission said on Wednesday that seizures by police and customs were failing to cut supply or reduce demand.
According to the report, the £5.3bn market was too "fluid" for the measures to have much impact, with, for example, drug dealers taking measures to reduce purity in times of short supply.
Some £380m was spent on cutting supply in England in 2005/06, and the study put the broader criminal justice costs from Class A drugs at over £4bn.
Half of the expenditure was said to have gone on tackling crack and heroin.
The report found that the number of Class A drug seizures in England and Wales more than doubled between 1996 and 2005.
But it added that more should be done to reduce the "collateral damage" of drugs on the community, by tackling gang violence and prostitution.
"Despite significant drug and asset seizures and convictions of traffickers and dealers, drug markets have proven to be extremely resilient," it said.
Co-author Tim McSweeney said there "is a consistent call for a better understanding of how drug markets operate and the role that enforcement can have in reducing the damage caused by them".
"We were struck by just how little evidence there is to show that the hundreds of millions of pounds spent on UK enforcement each year has made a sustainable impact and represents value for money," he said.
Former police chief constable David Blakey of the UK Drug Policy Commission said: "All enforcement agencies aim to reduce drug harms and most have formed local partnerships to do this, but they still tend to be judged by measures of traditional supply-side activity such as seizure rates.
"This is a pity as it is very difficult to show that increasing drug seizures actually leads to less drug-related harm.
"Of course, drug dealers must be brought to justice, but we should recognise and encourage the wider role that the police and other law enforcement officials can play in reducing the impact of drug markets on our communities."
A Home Office spokesman acknowledged that "enforcement in isolation is not effective".
Dods Parliamentary Communications Ltd