AN investigation has been launched after two patients managed to break into a medicine cupboard in a hospital's medium-secure mental health unit and steal tranquillisers.

The incident, at Calderstones Hospital, in Whalley, led to two men being taken to Burnley General Hospital's Accident and Emergency Unit for treatment.

Now hospital bosses at Calderstones are trying to work out what happened to make sure it doesn't happen again.

Staff today said the incident, earlier this month, highlighted the need for more staff, and warned: "It could have been much worse."

Calderstones chief executive Russ Pearce confirmed to the Lancashire Evening Telegraph that an incident had taken place in the West Drive unit of Calderstones -- the medium-secure part of the complex.

It is thought that a member of staff was handing out prescribed drugs to patients through a kitchen hatch.

He said initial inquiries suggested that the phone had rung in the kitchen, and the nurse on duty had gone to answer it, leaving the kitchen in the process.

Mr Pearce said: "The nurse thought the door would close automatically, but it had a wedge so that it did not close all the way.

"One male patient got into the kitchen and took a bottle of Chlorpronazine, a mild tranquilliser.

"When the nurse returned, the missing bottle was noticed and a search led to it being found with a patient.

"As a precaution, two male patients were take to Burnley General Hospital."

When asked how many pills the pair had taken, Mr Pearce said: "They did not appear to be suffering any major side effects."

One nurse, who works on the unit, said: "It has shocked us all because we know it could have been much, much worse.

"We're all so busy that mistakes will happen."

Supervision of patients has long been an issue at Calderstones after a series of break-outs by patients living in the medium-secure unit.

Earlier this year, the Lancashire Evening Telegraph revealed staff were being given maps of Whalley to help them find patients who went missing.