One in three drivers suspected of driving while 'over the limit,' but subsequently found to be below maximum permissible levels of alcohol, nevertheless tested positive for a range of drugs, reveals research in Injury Prevention.
The findings prompt the authors to call for routine drugs testing in all drivers who are suspected of being over the limit for alcohol.
The researchers base their findings on 2000 blood and urine specimens taken from drivers who had been stopped by police on suspicion of driving while 'under the influence' over a period of two years in Ireland.
Half of the specimens were below the maximum legal alcohol limit of 80 mg/100 ml for blood and 107 mg/100 ml for urine. The other half were all above.
But when analysed further, one in three samples below the legal limit, tested positive for a range of drugs. These drivers were also more likely to be taking a cocktail of drugs.
This rate was almost twice as high as that of drivers over the legal limit, one in seven of whom tested positive for drugs.
The drugs found included amphetamines, metamphetamines, benzodiazepines, cannabis, cocaine, opiates and the heroin substitute methadone. The most commonly found drug was cannabis.
Rates of testing positive for drugs were marginally higher among men than they were among women.
Based on the samples in the study, the authors calculate that almost 16% (one in six) of all drivers stopped and tested under suspicion of driving under the influence of an 'intoxicant' would test positive for drugs.
As blood alcohol levels rose, the likelihood of testing positive for drugs fell. But more than one in 10 drivers at least 2.5 times over the legal limit for blood alcohol (greater than 200 mg/100ml) also tested positive for drugs.
And among those with minimal blood alcohol levels, over two thirds tested positive for at least one type of drug, the findings showed.
Being under the legal limit for alcohol, being stopped in a city, stopped between 6 am and 4 pm or between 4 pm and 9 pm, and being under 35 years were all independently associated with drug taking.
Too little attention has been paid to the adverse effects of drugs on driving, but drugged driving can be as dangerous as drunken driving, say the authors.