A female prisoner was conveyed from Canberra's jail to the hospital on Sunday after suffering a medical emergency.
The nature of the emergency would not be revealed by ACT Corrections, however sources within the Alexander Maconochie Centre advised the woman had suffered from a drug overdose.
Her medical condition is unknown, and an internal investigation has been launched.
The emergency occurred as the Inspector of Correctional Services, Neil McAllister, was winding up a series of visits to the jail ahead of his first ACT "healthy prisons review".
A national health survey last year found that 89 per cent of people who entered Canberra's prison had been illicit drug users in the 12 months before they were put in jail.
A third of discharged prisoners said they had used illicit drugs while inside.