At least 10 per cent of people killed or injured in traffic accidents in the European Union are taking some type of drug, according to a new report.
The Health Research Board report also says there has been a four-fold increase since 1987 in the number of drivers killed who have traces of illegal drugs in their bodies.
But the report conducted for the European Monitoring Centre for Drugs and Drug Addiction says alcohol is "the biggest problem" in road accidents.
"Where involvement in traffic accidents is concerned, no drug has ever been found with a frequency that compares to that of alcohol," said the author, Ms Rosalyn Moran of the HRB.
Alcohol was found to be involved in 19 per cent of injurious accidents and 22 per cent of fatal accidents in the EU.
Ms Moran said alcohol has a "deleterious effect on a range of driving skills", including brake reaction time, collision frequency, steering responsiveness and risk-taking appreciation.
She said that the "lesser problem" posed by other drugs, such as benzodiazepines and amphetamines, is "still important". Benzodiazepines "may impair driving skills" in the first few weeks of treatment. Then the effects "may dissipate with continued use".
Ms Moran said cannabis "does not seem to significantly impair very basic perceptual mechanisms", but it "does impair more subtle aspects of perceptual performance such as attention and short-term memory, although these are typically observed at higher doses".
Methadone, the substitute for heroin, "does not result in sufficient driving impairments to merit users being designated unfit", particularly among experienced users, she said.
Amphetamines have "few effects" on cognitive functioning at lower doses, but at higher doses, "risk-taking increases and responses become inappropriate".
She said there are no laws in the EU defining illegal blood limits of illicit drugs or medicines and said there is "insufficient" evidence to define safe levels.
Advances in drug testing technology must be developed further, she said, and called on police officers to be trained in "drug-related impairment recognition".