Tories have branded the rise in prisoners receiving methadone as "extremely worrying" after it was revealed the number of inmates given the heroin substitute increased by more than a third in less than four years.
An audit carried out in Scotland's jails last month showed 1,679 prisoners were receiving the drug - 21.5% of the prison population.
The total number of inmates being given the drug is also 36.7% higher than it was December 2006 when a similar audit showed that 1,228 inmates were on methadone.