A heavily pregnant mother has been taken into police custody and questioned after admitting to using cannabis oil to treat her profoundly disabled three-year-old son's epilepsy.

Police seized the treatment, made form a non psychoactive form of cannabis oil, and questioned Cassie Batten of Mernda, north-east of Melbourne at Epping police station on Thursday afternoon.

The mother from Mernda, north-east of Melbourne, left the station with her partner Rhett Wallace following the questioning. It is understood no charges were laid.

Ms Batten declined to comment outside the station, saying "I can't say much at this stage".

The couple could face charges at a later stage of possessing a drug of dependence and introducing a drug of dependence into the body of another.