Five prisoners at HMP Glenochil were taken to hospital on Christmas Day - after drug misuse at the prison.
35 inmates were identified as being under the influence of drugs.
All prisoners are now safe.
The Scottish Prison Service says attempts to bring drugs into prisons are posing a "significant" threat.
Inmates staged a protest at HMP Glenochil last year.
They were unhappy about the amount of time they're allowed to spend outside their cells.