Using data from the National Highway Traffic Safety Administration on road deaths in 14 states, researchers also found that men and people driving at night were the most likely to have alcohol, marijuana or other illicit or prescription drugs show up on a toxicology screen after the accident.

Their results appeared in the journal Addiction.

"More than half of fatally injured drivers in the United States had been using alcohol or other drugs and approximately 20 percent had been using (two or more) drugs," wrote Joanne Brady of Columbia University and her colleagues.

Out of 20,150 fatally injured drivers in 2005 to 2009, 57 percent tested positive for at least one drug, including one in five who had multiple drugs in their system.

Alcohol was the most common drug, followed by marijuana and stimulants, including amphetamines.

Sixty percent of men killed while driving had drugs or alcohol in their system, compared to less than half of women. People who had an accident at night or on the weekend were also more likely to test positive than those driving on a weekday.

African-Americans and whites were equally likely to test positive after a fatal accident. Asians were much less likely to have drugs or alcohol in their system, while Native Americans were much more likely, the researchers said.

But the records couldn't show whether drivers had enough of a certain drug in their system to feel or act impaired or if prescription drugs were used incorrectly, experts said.

The researchers also didn't have data on the amount of the drug present in blood or urine samples, and cut-offs for levels that reflect impairment at the time of an accident aren't always clear, said Brady and her colleagues.

"There already is quite a bit of research that's probably going to continue to try and identify in drugs the point at which they are impairing," said Robert Voas, who studies alcohol and highway safety at the Pacific Institute for Research and Evaluation in Calverton, Maryland. He was not part of the study.

"With alcohol, the amount of alcohol is more or less directly related to the level of behavior impairment. The relationship of a drug in the body to the behavior of the driver is less direct and clear."

SOURCE: bit.ly/QnUohG

(Reporting from New York by Genevra Pittman at Reuters Health; Editing by Elaine Lies and Paul Tait)

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