A TTC driver charged with impaired driving Wednesday night was allegedly under the influence of drugs prescribed to him by his doctor, the employee's union says.

A 47-year-old man was arrested Wednesday after witnesses reported erratic driving to police.

Const. Hugh Smith says police did a sobriety test and determined there was enough evidence to arrest the driver for allegedly being impaired by drugs.

A statement from the Amalgamated Transit Union says the operator was not impaired due to alcohol or illegal drugs – he was on "doctor-prescribed medication he had been taking for several months."

The statement from the union also says the driver was not advised that his medication might cause impairment.

In March, a Toronto transit bus driver lost her driver's licence for three days following a roadside breath test.

The unidentified driver has been released from custody and is scheduled to appear in court on June 24th.

With files from The Canadian Press.