They found that 33% of girls had used illicit drugs compared with 40% in a similar survey carried out in 1995.

Although we must take the use of illicit drugs seriously, one should not be too panic stricken about it

Dr Patrick Miller, Alcohol and Health Research Centre

For boys the proportion dropped from 45% to 39.5% over the same period.

Drugs which were used less often included cannabis, solvents, amphetamines and ecstasy.

There was also a significant fall in the use of crack cocaine.

The only exception to this trend was heroin - although its use remains rare among teenagers.

Regional differences

There were some regional differences. Students in Scotland were more likely to have used cannabis and amphetamines.

The greatest use of solvents and glues was reported by teenagers from Northern Ireland.

Writing in the British Medical Journal, the researchers, Dr Martin Plant and Dr Patrick Miller say: "These reductions may be temporary, but they are striking."

Drug usage

Cannabis - 36.5% Glues and solvents - 18.6% Lysergide - 5.2% Amphetamines - 8.9% Ecstasy - 5.0%

Dr Miller told BBC News Online that educational initiatives may have helped to cut drug use. Many teenagers also had access to websites that detailed the bad experiences of their counterparts who had taken drugs.

But he also said: "It may just be a change in fashion."

Dr Miller said the research also showed that while 36% of teenagers had tried cannabis, only 7% had taken it on more than 40 occasions.

He said: "Although we must take the use of illicit drugs seriously, one should not be too panic stricken about it.

"In a lot of cases it is simply a bit of experimentation and does not go any further.

"However, that is not to say that for the few who do go on and use illicit drugs regularly that does not lead to serious problems."

Cyclical phenomenon

Harry Shapiro, director of communications for the drug information charity DrugScope, said the research was echoed by other similar studies.

"We can say there has been a kind of levelling off, or even a decline in some types of drug use - certainly we don't appear to be experiencing the sky rocketing statistics of the early and mid-nineties.

"Why that should be is not easy to explain. The most likely explanation would be that drug taking tends to go through cycles.

"This may be due to the fashion swings of youth culture - they do not want to be like the previous generation."

A study by the Office for National Statistics, published earlier this week, found that there had been a slight increase in drug usage among schoolchildren aged 11 to 15 between 1998 and 1999.