The drug is usually used to treat a heroin addiction.
13% of prisoners in the midlands are on methadone, which is normally used to treat an addiction to heroin.
4% of inmates at Portlaoise Prison are given the substance, while that's 9% in the Midlands Prison.
That compares to an average of 17% in all Irish prisons, according to details released under the Freedom of Information Act.
Dr. Greg Kelly doesn't agree with such widespread use of the substance.
Dr. Kelly, who has previously worked Castlerea Prison, says methadone should only be given to prisoners for a very short period: