The purity of cocaine powder was as low as 2% in one area
A decline in the quality of drugs is encouraging takers to use a wider range of substances, a report suggests.
But the study from UK information charity DrugScope warns treatment services may not have the capacity to help so-called "poly-drug" users.
The information was compiled from data provided by 70 UK drug services, police forces and drug action teams.
It confirms findings of a drop in the purity of cocaine but also suggests the quality of other drugs has fallen.
With drug prices stable or in decline, the 2009 edition of DrugScope's annual Street Drug Trends Survey says this may be accelerating a longer-term trend of people taking a variety of substances together or separately.
Average street drug prices
Herbal cannabis (good quality) /oz
£40 per quarter oz*
Herbal cannabis (standard) /oz
£31 per quarter oz *
Resin cannabis /oz
£21 per quarter oz*
Cocaine per gram
MDMA powder per gram
Ketamine per gram
Amphetamine per gram
Diazepam per 10mg tablet
*quarter oz is most common amount dealt on street drug market
Researchers say that because there is a more varied menu of drugs, users are less bothered about individual quality.
Dealers are getting more greedy
Gary Sutton Release
It said the quality of heroin, ecstasy and illicit tranquilisers, was significantly lower.
The report raises concerns that drug treatment services may not have the capacity to help poly-drug users - in particular those who use drugs which have only recently emerged, such as Ketamine and GHB.
In 17 out of 20 areas studied by researchers, a drop was recorded in the quality of cocaine powder and crack.
In Bristol, police reported seizing cocaine powder with purity levels as low as 2% and a Suffolk drug squad officer told the survey that the purity of crack in Ipswich had dropped from 60% to 20% within the space of a few years.
Some 12 out of the 20 areas reported a decline in heroin quality, while a majority also recorded a fall in the MDMA content in ecstasy pills.
Mr Barnes said there had been a long-term trend towards users combining a range of drugs, but suggested that the low-quality substances "dominating" the market may have accelerated this process.
He added: "While overall levels of drug use have remained relatively stable in recent years, the range of substances appearing on the radar of drug services and enforcement agencies appears to be increasing.
"The fact that older teens and young adults are increasingly combining substances including ketamine, cocaine, cannabis and cheap high-strength alcohol is particularly concerning."
In Ipswich and Middlesbrough, researchers found that crack users were turning to alcohol and black market pharmaceuticals, whereas in Newcastle some were buying powder cocaine instead because it was cheaper.
The report also warned that an increasingly "junk food" drug market was leading users to turn to less well-researched alternatives.
In 18 of the 20 areas covered by the survey, ketamine was reported as being used by a growing number of young people.
And for the first time in the survey's five-year history, some drug services expressed concerns about the use of the so-called "legal highs" such as GBL and mephedrone.
Gary Sutton, head of drug services at the charity Release, said purity levels reflected the state of the drugs economy.
"Dealers are getting more greedy, or, to put it more accurately, increasing the risk premium," he said.
"There seems to be a longer chain from importer to the street and users, especially cocaine buyers, seem happy to buy lower grade substances.
"Most heroin users will complain about their gear, but most will buy it anyway."
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