Researchers have found that smokers who take up the habit again after a period without using tobacco find that their ability to reason improves.

They also found that smokers who are deprived of nicotine find it difficult to concentrate, and that going without cigarettes magnifies the craving for nicotine.

The research team from the National Institute on Drug Abuse, Baltimore, USA, tested a group of hardened smokers to ascertain the impact of going without nicotine.

Lead researcher Dr Stephen Heishman, writing in the journal Nicotine and Tobacco Research, said: "It is likely that craving and impaired concentration, in part, function to maintain smoking in nicotine-dependent individuals and to increase the probability that a smoker attempting to quit will relapse."

Computerised tests

Once addicted it can be difficult to quit

The researchers studied the effects of smoking and abstinence on 20 smokers who completed two computerised tests before and after smoking one afternoon and again after being deprived of cigarettes for 18 hours.

The participants typically smoked 23 cigarettes a day and had been smoking on average for more than 17 years.

In one test, smokers were instructed to find specific pairs of letters in a sequence of 20 other letters.

The other test presented them with a series of letter pairs and statements and was designed to measure their ability to reason.

Responses revealed that the smokers' cravings for cigarettes increased significantly over the 18-hour abstinence.

After they had been allowed to smoke again, however, their cravings subsided to the levels observed at the beginning of the study.

On the letter-search test, the smokers took longer to complete the task when deprived of cigarettes, but their speed improved to baseline levels after smoking.

On the test of logical reasoning, however, performance did not decrease after abstinence, but actually improved after smoking.

Clive Bates, director of Action on Smoking and Health, said the study showed the huge physiological impact that nicotine withdrawal has on the body.

He said the apparent boost in reasoning skills when smokers took up the habit again after a period of abstinence could be a result of the relief experienced by getting another nicotine hit, and was likely to be a very temporary effect.