The Metropolitan Transit System board was alerted to the drivers, as well as another employee, being put on leave Wednesday, KGTV, San Diego, reported.
"The source of the illness has been traced back to baked goods distributed by one fellow employee ... all employees involved in this incident are on leave pending drug testing," said a memo sent out to board members from MTS CEO Paul Jablonski.
An MTS employee brought brownies to the MTS bus terminal on Sunday, sources told KGTV. The brownies, baked by the employee's roommate, were apparently laced with marijuana, unknown to the employee, the report said.
Later in the day, the four bus drivers complained of dizziness and had to pull over on their respective routes.
Two of them -- both female -- were hospitalized, KGTV said.
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