Patients with mental health problems should not have to pay for prescription drugs, according to campaigners.
Many patients are also seeking treatment outside the NHS
The mental health charity Mind has called on the government to scrap the charge - currently £6.30 - for these patients, saying it is unfair.
It follows a survey which suggests that many people with depression and similar conditions are paying hundreds of pounds a year for drugs prescribed for them by their GP.
The survey, carried out by Mind and the magazine Health Which?, suggests that almost half of patients with mental health problems have paid for prescription drugs out of their own pocket.
It found that these patients were spending an average of £37 each month on prescription drugs, with a small number running up monthly bills of £100.
The survey of 455 people with mental health problems found that many were also spending money on treatments unavailable on the NHS. These include complimentary therapies, counselling and psychotherapy.
Over half said they spent in excess of £61 a month on such treatments. One in five said their bills ran to at least £100 each month.
Most of these said the failure of the NHS to provide such care had probably delayed their recovery.
The NHS is selling people with mental health problems short
Campaigners said people with mental health problems should be treated in the same way as patients with certain chronic diseases, who do not have to pay prescription charges.
Richard Brook, chief executive of Mind, said: "The NHS is selling people with mental health problems short.
"People with enduring mental health problems should not be paying out of their own pockets for essential care and treatment."
Kaye McIntosh, editor of Which?, also called for the rules to be changed.
"If you break your leg and need physiotherapy you don't have to pay for it.
"If you have diabetes or epilepsy you don't have to pay for prescriptions. People with chronic mental health problems shouldn't be penalised - especially as financial stress is known to put pressure on our mental well being."
Mind is hoping people will send postcard petitions to Prime Minister Tony Blair, as part of a campaign to persuade the government to ensure people with mental health problems get the care they need free on the NHS.
The Department of Health defended its policy.
A spokeswoman said: "Prescription charges have been part of the NHS for years. We know that around 85% of people with severe mental illness do not pay for their prescriptions - this is based on their ability to pay rather than the nature of their illness.
"Our policy is to give priority to helping people who may have difficulty in paying charges, rather than extending the exemption arrangements to people suffering from other conditions."
But Liberal Democrat health spokesman Paul Marsden said:
"The government refused to support the Liberal Democrat private members' Bill on prescription charges that I presented to parliament last year.
"It aimed to scrap prescription charges for all chronically ill people, including those suffering from a variety of mental illnesses such as Parkinson's Disease and Alzheimer's."
He added: "The NHS is supposed to be free at the point of delivery but under Labour many patients with long term illnesses are having to pay for basic drugs.
"It is a disgrace that in the 21st century Labour allows this situation to persist."