Narcotics

"It is forbidden to drive if your blood contains any substance classed as a narcotic. This does not apply if the medication has been prescribed by a doctor."

Driving Licence Book, 19th Edition, page 213

Allowed: Permitted to drive if you have narcotic medication in your blood that was recommended by a doctor and you are capable to drive safely.
Not Allowed: You are not permitted to drive if you have medication in your blood that contains a narcotic substance prescribed by a doctor and you are unable to drive safely. Eg: Sleep medicine, Muscle relaxers