A profoundly handicapped teenager was drugged with morphine by her foster carer who has pleaded guilty in the County Court in Ballarat to one count of recklessly causing serious injury.

Susan Melinda Lizzul, 54, admitted on Wednesday that she gave the powerful opioid to the then 13-year-old girl in 2016, when the child lived at a Sebastopol home and required 24-hour care for a significant cognitive impairment and several other medical conditions.

However, Ms Lizzul did not reveal to the court why she administered the drug to the teenager, who has been in a constant vegetative state since birth and suffers up to 50 seizures a day.

The victim was taken by ambulance to the Ballarat Base Hospital on November 16 , 2016, after being found in a "fluctuating conscious state".

Dr Mark Nethercote from Ballarat Base Hospital said the girl would have died without urgent medical attention, according to a summary statement by the Office of Public Prosecutions.