Britain's biggest doorstep lender has been criticised after a BBC investigation found it had loaned thousands of pounds to a woman with a serious mental illness.
The woman, a diagnosed schizophrenic, was given multiple loans.
The Citizens Advice Bureau accused the firm, Provident Financial, of lacking social responsibility.
Provident said in a statement it makes every effort to ensure borrowers are capable of making informed decisions.
During the undercover investigation by the BBC's Panorama programme, an agent acknowledged that the customer was "not all there", yet the company had been selling her loans for a number of years.
The Office of Fair Trading's guidelines for creditors state that borrowers who may be particularly vulnerable by virtue of their age, health or disability should not be targeted or exploited.
Gillian Guy, chief executive of the Citizens Advice Bureau, a charity which offers help to people in debt, said: "I call into question… the motivation to keep exploiting people who clearly cannot be held responsible for their own decisions in that situation."
Provident Financial, which has its headquarters in Bradford, has agents in most towns across the UK and last year reported pre-tax profits of £127m in its home credit division.
In a statement, the company said it has strict policies to prevent loans being advanced to anyone it believes does not have the mental capacity to understand the terms of a loan.
Provident said it recognises that "with 1.8 million customers it will not always get it right and if it makes a mistake it works hard to put matters right".
The mother of the customer with mental health problems told the BBC that anyone meeting her daughter would be able to see that she was unwell.
"Anyone in their right mind would know she's got an illness," said the woman's mother, who asked not to be named in order to protect her daughter's identity.
"And then she'll talk as though she's got three or four voices running round her head sometimes. Well naturally you'd know there's something wrong with somebody."
In another case, a Provident manager acknowledged that she did not believe the customer she was visiting could look after herself.
The customer told the BBC that said she has had loans with the company for the last seven years and, because she has a private pension, she believed she could afford to pay them back.
Provident said in their view, the woman was capable of making informed decisions about her finances.
Her sister, however, told the BBC that she did not believe she should have been issued with the loans.
"They would clearly have been able to see just by looking around the house that she is vulnerable."
When asked what she thought of the doorstep loan agent who had approved her sister's loan, she said: "Disgusted, they have taken advantage of her."
Gillian Guy of the Citizens Advice Bureau said the agents were a product of the incentive system put in place by the company whereby they are rewarded not only for collecting on outstanding loans, but for selling new ones.
"I do not blame the agent for that because that is the mill they are in."
Provident Financial said in a statement that it is properly regulated and adheres to the Office of Fair Trading guidelines on responsible lending.
It added: "Provident takes great care to ensure that it only lends amounts appropriate to the personal circumstances of each customer."
Panorama's Undercover: Debt on the Doorstep, BBC One, Monday, 1 October at 20:30 BST and then available in the UK on theBBC iPlayer.