A Toronto Transit Commission bus driver charged with impaired driving Wednesday was under the influence of drugs prescribed by his doctor, according to the transit workers’ union.

“He was apparently affected by doctor-prescribed medication he had been taking for several months,” Bob Kinnear, president of the Amalgamated Transit Union Local 113, said in a statement.

“He had not been advised that his medication might cause impairment. There had been no prior indication of impairment from the medication. A medical investigation is underway,” he said.

Zvi Domb of Toronto received one charge of impaired driving after police responded to a call from a citizen near Senlac Rd. in the Yonge St. and Finch Ave. area, at about 8 p.m., Const. Hugh Smith said.

“When we got there, the bus was already stopped and pulled over,” he said, adding that police didn’t know whether the bus was in service or on its route.

There were no passengers on the bus, Smith said.

The driver, 47, failed a field sobriety test administered on site. He was arrested and charged.

Smith said the driver was released Thursday morning and cannot operate a motor vehicle until his court appearance on June 24.

The TTC is conducting its own investigation. It’s not yet known if the driver saw a supervisor during the course of his shift, said TTC spokesman Brad Ross.

Wednesday’s was the third incident in which a TTC driver has been charged for impaired driving since June 2008. In March, a TTC bus driver was suspended after failing a breathalyzer test administered by police. The bus had been pulled over after passengers called police to complain about the driver’s behaviour.

Ross wouldn’t say how many incidents there have been in which no one was charged.

The TTC began drug and alcohol testing for its new employees in February. Since then, 11 of 197 tests have come back positive, he said.

In August, the TTC will implement the remainder of its new fitness-for-duty policy, which requires employees with a record or suspicion of workplace impairment to undergo testing.

Employees are still being trained in the new procedure and policy, which was approved by the transit commission nearly two years ago, said Ross.

Although TTC staff recommended that all employees be subjected to random drug and alcohol testing, the councillors on the transit commission rejected the move.

The policy affects all safety sensitive positions at the TTC, which covers the majority of employees.

“Our policy is you must be fit for duty and not impaired in any way,” including fatigue, drugs and alcohol, said Ross.

“We do have 12,000 employees. Whenever these incidents come to our attention they are considered isolated incidents. We don’t feel there is a systemic problem,” he said.

Workers on Toronto’s transit system have faced a barrage of criticism from the public since January, when a fare collector was photographed sleeping during his shift at McCowan station.

With files from Gloria Er-Chua