TORONTO -  It's known that taking certain medications and getting behind the wheel can lead to trouble, but a study of older drivers and antidepressants is adding more clarity to the subject.

Drivers aged 65 and older who are taking antidepressants combined with another psychotropic medication are at increased risk of being at fault in a motor vehicle accident, the study found.

Antidepressants alone did not lead to a heightened risk of a motor vehicle crash.

The research, published Wednesday in the American Journal of Geriatric Psychiatry, used information from Ontario transportation and health-care databases from Jan. 1, 2000 to Oct. 31, 2007.

Co-author Dr. Mark Rapoport said researchers looked at the at-fault status of the crashes, which hadn't been done in a lot of previous studies.

"Antidepressants did increase the risk of crash significantly but ... that increased risk was only restricted to at-fault crashes and only restricted to crashes where there was another strong centrally acting medication on board as well, so a benzodiazepine or a strong anticholinergic medication."

One of the main symptoms of depression is insomnia, explained Rapoport, an associate professor of psychiatry at the University of Toronto.

"And so benzodiazepines or sedative hypnotics are used for the treatment of insomnia, and so very often patients are prescribed both -- and in that context there was a 23 per cent increased risk of crash."

Examples of benzodiazepines are alprazolam (Xanax), clonazepam (Rivotril) and lorazepam (Ativan).

Meanwhile, some drugs used to treat gastrointestinal and respiratory disorders have potent anticholinergic properties.

The elevated crash risk seemed to be apparent for the first three or four months after a person first began taking the antidepressant, Rapoport said, and it returned to baseline after that.

"Our study wasn't set up to figure out why this happened," he said. "And one of the things that we don't know is that it's possible that the antidepressant started working by that time and that it wasn't the antidepressant at all, but the depression that was still active that was causing the crashes," he said, adding that "this is just a theory."

There are some limitations to the study.

For instance, the researchers didn't look at the dose of the antidepressants, and they don't know about the possible effect of dementia, he noted.

Rapoport said much more research is needed on the effect of depression and antidepressants on driving.

The study said antidepressants are one of the most commonly prescribed psychotropic medications for older adults, with about one in 10 seniors receiving a prescription.

Dr. Donald Redelmeier, a co-author, said elderly drivers often have additional chronic illnesses that increase the risk of motor vehicle crashes.

"Special attention is needed when complex patients are taking multiple medications, since impaired driving creates risks of the driver and other vulnerable road users," he said in a statement.

Rapoport said doctors and pharmacists should warn patients of the crash risk associated with the first few months of taking an antidepressant, especially if the patient is elderly and taking other medications.

The study was a collaboration of the Institute for Clinical Evaluative Sciences, Ontario Ministries of Health and Transportation, and Sunnybrook Health Sciences Centre.