A charity has voiced fears at the rising number of drug users taking heroin and crack cocaine together.
A number of cities are said to be seeing rises in speedballing
DrugScope looked at 20 UK locations and found many users were "speedballing" for a stimulant-sedative effect.
It warned the practice could lead to rising crime, saying, on average, speedballers have three times as many convictions as heroin-only users.
It found they spent an average £500 a week on drugs compared with £110 for heroin users.
Martin Barnes, the chief executive of DrugScope, said the risks associated with speedballing were very serious.
"Evidence shows there is a greater risk of overdose, you're much more likely to inject more often, and there are greater links to crime.
"It's a combination of very powerful sedative and stimulant drug, so it does create a sort of different effect, but it really is a very risky and dangerous combination of drug."
In presenting its study, DrugScope highlighted a survey of 100 drug addicts by Dr Russell Newcombe at Manchester drugs charity Lifeline.
His research revealed speedballing was the main way to take drugs for 80% of those interviewed - the figure was 25% a decade ago.
Harry Shapiro, editor of the charity's Druglink magazine, said: "Although speedballing isn't a new phenomenon, it is clearly on the increase and if this trend continues, it will be bad news for attempts to reduce the spread of injecting-related diseases and the number of drug overdoses."
UK STREET DRUG PRICES
Belfast has the priciest cannabis resin, crack and heroin
Gloucester has the cheapest cocaine and herbal cannabis
Cardiff has the cheapest ecstasy and the most expensive cocaine
The trend also created the potential for increases in overdoses and viral infections, the charity said.
Speedballing was on the rise in Newcastle, Sheffield, Manchester, London, Bristol, Nottingham, Ipswich and York, DrugScope found.
In Liverpool and Ipswich it was reported that dealers were offering free crack with heroin.
Eleven of the 20 areas survey also showed a significant anabolic steroids problem, involving males as young as 16 attempting to get a muscular physique.
John Belushi and River Phoenix both died after taking a speedball
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Red Hot Chili Peppers frontman Anthony Kiedis has admitted speedballing
The risks of taking such drugs include reduced sperm count, kidney and liver problems, high blood pressure and increased aggression.
There is also the risk of contracting diseases such as HIV for those who inject steroids.
DrugScope described the trend as "extremely worrying".
Blackpool, London, Birmingham, Middlesbrough, Nottingham, Torquay, Cardiff, Manchester, Portsmouth, Luton and Newcastle were among the places found to be suffering rising steroid abuse.
DrugScope surveyed 80 drug agencies and police forces for its study.