The company running the bus will also be probed to find out if the executives knew the driver took drugs.
Speaking about the DIT's measures to deal with staff misconduct, department director-general Theerapong Rodprasert said wrongdoers will be warned and may have to undergo retraining.
If they repeat the misconduct, their licences will be revoked and they will be banned permanently from driving public vehicles, he said
Mr Theerapong said the DIT will also attempt to tackle the problem in the early stages, by toughening the public driving licence exam and screening out unsuitable candidates.