THREE prison officers were taken to hospital after inhaling second-hand smoke from the "zombie" drug spice in a West Lothian prison.
The Scottish Prison Service (SPS) said two of the officers were treated after becoming unwell after the incident at HMP Addiewell on Saturday.
The drug is reported to be a growing problem in prisons across the UK.
Spice, also called the zombie drug because of the way it affects users, is the name for a range of new psychoactive substances which were banned banned in May 2016.
Previously called "legal highs" they were originally designed to mimic the effects of cannabis, but affect the brain differently - and can have severe debilitating effects.
Steve Farrell, from the the Community trade union which represents members at HMP Addiewell, said two officers had fallen "seriously ill", but were now recovering and in a stable condition.
He told BBC Radio's Good Morning Scotland programme: "It's been reported to us for a couple of months from the staff and from our members in Addiewell that the illegal highs and Spice in particular is causing some real concern."
Mr Farrell said the issue had been highlighted to union officials more in HMP Addiewell than any other Scottish prison in recent months.
He added: "I've been in the prison service a long time. We started with cannabis, it then led to heroin and no unfortunately we're onto things like Spice.
"This in particular is probably more unknown in terms of the affects and medical affects than other class A drugs such as heroin and cocaine."
According to the SPS, Addiewell is designed as a "learning" prison, where offenders can address their behaviour and the circumstances which led to their imprisonment.
A spokesman for the SPS said: "We can confirm three staff were taken unwell while on duty on Saturday and taken to hospital by ambulance.
"We work closely with the police and NHS to tackle drugs, which are a challenge across the whole prison estate."
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