Anti-smoking campaigners say the research indicates nicotine is as powerful a drug as cocaine.

However, drug users preferred the "high" they experienced from cocaine.

The experiment found that the addicts were easily confused as to whether they had been injected with nicotine or cocaine.

The findings are published in the Journal of Pharmacology and Experimental Therapeutics.

Injections

The study involved 10 drug users each of whom smoked and had a serious cocaine habit. They abstained from smoking for at least 8 hours before each session.

Researchers injected each of them with doses of either nicotine or cocaine. The volunteers were not told what they were receiving.

Smokers were tested

When given a high dose, the subjects were more likely to identify nicotine as either as a stimulant drug or an opiate such as heroin.

When they recognised the substance as a stimulant, three-quarters mistook nicotine either for cocaine or amphetamine.

Nicotine was rated as a more powerful drug than cocaine at high doses, but not as pleasant.

The study was performed by Dr Roland Griffiths, Hendree Jones and psychiatrists from Johns Hopkins University School of Medicine.

It is the first to make a direct comparison between the effects of nicotine and cocaine on drug users.

Laboratory experiments had previously shown that nicotine has similar effects on the body to stimulants such as cocaine and amphetamine.

These effects include increases in heartbeat rate and blood pressure.

Nicotine was also found to produce similar changes in the chemical balance of the brain.

Rush

In the tests, nicotine was faster acting than cocaine and the drug takers reported that it produced more "rush" and "high" effects.

However, the nicotine sensation was perceived as less pleasant than cocaine at high doses, making the volunteers feel "jittery".

In their paper, the researchers said: "It is interesting that when subjects were asked to identify the type of stimulant they had been administered, subjects usually identified both cocaine and nicotine as being cocaine or amphetamine and almost never identified either drug as being nicotine."

The anti-smoking pressure group Action on Smoking and Health (ASH) said the findings demonstrated how strong a drug nicotine was.

Dirty drug

Director Clive Bates said: "We know tobacco is powerfully addictive and that nicotine affects the brain in subtle ways that are similar to hard drugs like cocaine and heroin.

"We tend to think of nicotine as a soft drug, but this research adds weight to the idea that it is more like a hard drug, albeit one that is legal and in widespread use.

"A big difference between cocaine and nicotine is that tobacco users take their nicotine in a cocktail of 4,000 toxic chemicals.