As of February 21, 34 of the 85 inmates at Jurby prison were in custody for drug-related offences, and two for alcohol-associated crimes.
Home Affairs Minister Juan Watterson has revealed the figures in the House of Keys, in reply to fellow Rushen MHK Lawrence Skelly.
Meanwhile, in answering a supplementary, Mr Watterson admitted substances did find their way into the prison, but said measures were in place to tackle the problem.
He said one issue was with prisoners making their own alcohol, which could be done in any enclosed space:
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