Preston staff found the drugs were being smuggled into the jail via the prisoners’ mail, after the paper used to write the letters was soaked in the substance.
The notes could then be ripped up and smoked by the inmates.
In a bid to crack down on the problem, the category B men’s prison began photocopying all mail and keeping the originals locked away.
According to an annual report by the Independent Monitoring Board, the move has produced positive results – with not a single ambulance call-out needed for a prisoner under the influence since the scheme began in January.
The report said: “The Board’s analysis has clearly identified the effectiveness of this precaution in a directly correlated reduction in reported incidents of use of PS (psychoactive substances).
“Since the photocopying was introduced there have been no ambulances called to take a prisoner to hospital under the influence, resulting in savings to the NHS and improvements to prisoner welfare.”
The watchdog admitted the move might not eradicate the problem completely – adding that prisoners would find new ways of getting hold of the drugs – but said it had “demonstrably reduced the availability of PS within the prison”.
The report suggested the use of drug testing devices in prisons to scan incoming mail could prove a more cost-effective and less labour-intensive solution in the long term.
Read the report in full here.