Street cocaine has long been diluted, but now the cutting agents themselves have spawned a black market. Mild anaesthetics, found in sunburn and first-aid treatments bought at any chemist, are the latest substances being sought by drug dealers.
They call it the "bash" or "smash" trade and in the current financial climate it is one cut that is netting some people a small fortune.
The black market trade of selling legal chemicals to drug dealers to be used as so-called cutting agents has become a highly lucrative trade of its own, a BBC Radio 5 live Breakfast investigation has found.
Legally imported in bulk, the substances are mixed with the illegal Class A drugs cocaine and ecstasy, to maximise dealers' profits. Demand is high because they mimic the effects of the drug they are being mixed with - fooling customers into thinking they are getting better quality merchandise.
"The mark-up is starting to get as high as the likes of the coke," says one ex-drug dealer from Liverpool, who spoke anonymously to the BBC. "The money people are making off the cutting agents, it's through the roof.
"That is the big market now. If you can get a really good cutting agent, people will pay through the roof so they can put three times as much into their own stuff, so they can make three times as much profit."
The rise in these mild anaesthetics goes hand-in-hand with a drop in the purity of illegal drugs. Cocaine and ecstasy are "weaker" than at any time since the authorities started purity tests more than a decade ago. The Forensic Science Service (FSS), which tests drugs for the police, found almost 39% of the cocaine it tested last year was less than 10% pure - more than double the year before.
Drugs have always been cut with substances by dealers to make as much profit as possible. But whereas caffeine and glucose used to be the cutting agents of choice, the emphasis is now on anaesthetics.
The substances - such as benzocaine and lidocaine - are legal and imported from China and other countries. They are used in the UK as a dental anaesthetic and as an ingredient in first-aid ointments, throat sprays and sunburn remedies.
They are so popular because they closely mimic the physical sensations associated with cocaine, such as a numbing of the mouth or nose, says Dean Ames, a leading drugs scientist at the FSS.
This convinces drug users they have got a good product and massively increases the dealers' profit margin, say drug information groups.
Quick and easy
"It's a savvy way for dealers to cover the fact that the purity of drugs like cocaine is getting less and less," says Martin Barnes, the chief executive of DrugScope. "If there is a numbing of the tongue and nose people think the cocaine they are using is purer than it actually is.
"They want people to think the drugs are better quality because they want them to come back for more."
The demand for this type of cutting agent is creating a lucrative underground market in itself.
Police say the substances are being bought for a little as £10 a kilo, if ordered in bulk, and sold on to dealers for up to £300. The profits are getting so good some criminal gangs have now stopped selling illegal drugs to focus on trading the legal chemicals. And increasingly individuals looking to make a quick profit are ordering and selling from home.
"An individual was found to be selling what could be used as cutting agents from his garage on a housing estate in vast quantities, advertising in local papers and also on the internet," says Mick Matthews, Deputy Chief Constable of Gloucestershire Police. He is also the Association of Chief Police Officers' specialist on the cocaine industry.
"You can see a massive mark-up on the imported substances. The mark-up again from the cutting agent being added to an illegal substance, well that just goes through the roof.
"A kilo of cocaine, current price around £50,000, if you double that kilo with benzocaine and you have two kilos the dealer could then probably knock that out for something in the region of £80,000 to £90,000. So as you can see the profit margin on their initial outlay is significant."
The chemicals are easily available on the internet, but drug dealers are not going to buy them themselves and leave a credit card trail for police to pick up on - hence the middleman.
While they are not illegal to import, it is illegal to supply them to the underground drug trade. The Serious Organised Crime Agency (Soca) is now specifically targeting those dealing in the substances. If importers are unable to explain why they want such large quantities, the drugs are incinerated. Conspiracy laws are also being used to prosecute people.
Earlier this year, Soca seized 83 barrels (two tonnes) of benzocaine in just one week. The haul represented one-fifth of the UK's annual legitimate use of the drug. The importers were unable to explain why they needed it, so police disposed of it.
But the boom in the trade could have serious health implications for the people who are using the drugs, including liver and heart problems. The chances of someone taking an overdose also increases.
"Any drug is harmful, however pure it is," says Mr Barnes. "But variations in purity due to cutting agent being used increases the risk of an overdose. People just do not know what strength of drug they are taking."
Police also believe it could have other knock-on effects.
"Officers don't always know what they are dealing with," says Deputy Chief Constable Matthews. "The normal effects of heroin, cocaine, ecstasy, they are pretty well recognisable for the police.
"When it is mixed with something not known that can have a completely different side effect, that changes the temperament of the situation. It can change the behaviour of the individual - that has to create further risks."