The head of a psychiatric hospital admits more than one in seven patients takes illegal drugs on the wards.
Illegal drugs can make patients worse, health experts say
In recent weeks patients have smoked crack cocaine inside Chase Farm Hospital, in north London, while others have smuggled cocaine inside fast food.
Meanwhile, a BBC survey suggests there is a laissez faire attitude to drug use in NHS mental health trusts.
Some said they would not automatically call the police if they discovered drugs dealers on their premises.
There are strong links between mental health problems, the taking of illegal drugs and violence, with one major study finding that psychiatric patients with drugs problems kill up to 32 people every year.
But Oliver Treacy, borough director for mental health services for Barnet, Enfield and Haringey Mental Health Trust, admitted: "It is quite common that patients will use drugs on wards.
"It can make patients worse and it can actually introduce a sense of desperation and violence."
He estimated that up to 14% of patients in some wards at Chase Farm were currently taking drugs on the premises.
That is a reduction from a high of 20% in recent years, a figure which had prompted extra security measures, said Mr Treacy.
"It prompted us to address this issue and bring in procedures and policies that bring it to the situation we are in today, which is the impression that we are managing the situation."
Human rights legislation prevents hospitals from randomly searching patients so Chase Farm has brought in sniffer dogs to regularly sweep the wards.
Drugs workers helping in-patients at the hospital said one patient recently took crack cocaine on the ward.
One nurse said it was difficult to stop all illegal drugs entering the hospital.
Samuel Ankara said: "One patient was using a bed sheet to drop out of his window to pick up drugs from a dealer below, then another one tried to bring in drugs hidden inside a kebab."
There are no national figures showing how widespread illegal drugs are on psychiatric wards.
However, one study in north London concluded that almost half of the patients in some hospitals continued feeding their habit on NHS property.
Peter Phillips, a former nurse, spent several months on wards in Camden and Islington, London.
He found that almost half the patients in hospitals admitted to taking illegal drugs before admission, and of these, the great majority, 83%, admitted to drug use on the wards.
Typical of the inpatients on wards who continue to buy and use illegal drugs is Chris, who is currently detained on a ward in London after being diagnosed with bi-polar depression, which means he suffers from mood swings.
The 24-year-old, who has three children, has been in and out of psychiatric wards since 2001.
He says his condition is linked to his drugs use.
"I have seen patients in other wards and they're meant to be mentally ill and they doing crack, but I don't get involved in that."
Chris, whose name has been changed for legal reasons, admits to smoking cannabis on the ward.
"I've smoked cannabis in the smoking room and the staff noticed but they let me get away with it."
Experts say the problem is not helped by the fact that different NHS trusts have different policies on dealing with the issue.
BBC Radio's Five Live Report conducted a sample survey of some dozen mental health trusts around the country to find out what their policies were when drugs are found on wards.
Asked what they would do if they found patients dealing in drugs on the ward, five NHS trusts said they would not automatically contact the police.
Asked what they would do about patients found in possession of drugs, six trusts said they would not automatically call the police.
Asked finally what they would do if a visitor was suspected of smuggling in drugs, four trusts said again that they would not automatically call the police.
Marjorie Wallace, Chief Executive of the mental health charity SANE, speaking on BBC Radio Five Live, said: "We have been warning the Government for five years that by closing beds and diverting scarce resources into the community many in-patient wards have become neglected. "It is all too easy to be lured into the illicit drug culture by fellow patient drug-dealers and users.
"In psychiatric units, boredom can trigger violent outbursts, and instead of providing places of safety and treatment, patients and staff are subjected to an atmosphere of fear."
The Five Live Report: Drugs on the Brain can be heard on Sunday 14 May at 1000 BST on the Julian Worricker Show on BBC Radio Five Live.