A survey has shown high levels of illegal drug use among inmates in South West prisons.
Statistics obtained under the Freedom of Information Act, revealed almost 25% of inmates gave positive tests.
Cannabis is the most commonly used drug, but substantial numbers of inmates are taking opiates like heroin.
A Home Office report said the prison service has developed a substantial programme of drug treatments to tackle the problem.
The figures for last year showed Dorchester prison in Dorset had almost 25% of random drug tests proving positive.
Dorset's HMP Verne on Portland had 15%. Dartmoor prison in Devon had 10%, with Exeter prison just under 10%.
The figure for HMP Portland in Dorset was just over 5% positive testing.
Channings Wood prison near Newton Abbot in Devon had the lowest result, with just over 4%.