Yet nothing could have prepared Sudano, 72, for the nightmare that was to follow and the discovery of a trail of greed and fraud that went right to the heart of the medical profession.

She was astonished to discover that her trusted GP had been using her as a guinea pig by giving her tablets which had not been medically approved. Worse still, he was being paid to do so by a pharmaceutical company.

An investigation by The Observer has revealed that many doctors are risking their patients' health by subjecting them to medical trials without their knowledge.

Sudano's ordeal began when Adams took a blood test and asked her to return the next week. She had been living at Letchworth in Hertfordshire since arriving from Italy 50 years earlier and she trusted British doctors.

Over the next few weeks Adams asked Sudano to return for more blood tests. By the end of the second month her arms were black and blue. Her son, Joe, said they looked as though they had been slammed in a door. She said: 'At one point I asked if he was selling my blood.'

Little did Sudano know how close to the truth her comment was. On her last visit, Adams took a bottle of pills from the top drawer of his desk and suggested Sudano take one a day. While she thought it strange she wasn't being given a prescription, she trusted her doctor.

But within hours of swallowing the pill, she could hardly walk because she was so dazed. Her face had swollen up badly and she was in considerable pain.

She stopped taking the tablets and complained to Hertfordshire Health Authority. This sparked an investigation that led to one of the largest cases of medical research fraud ever uncovered in Britain.

It emerged that over the previous five years Adams had earned more than £100,000 from drug companies, including the European giants AstraZeneca, GlaxoSmithKline and Bayer. They were all paying him to test their new drugs on his patients. Like Sudano, many of his patients had never given their consent and had no knowledge they were being used as human guinea pigs in a medical trial. Patients with no symptoms were given drugs and others who needed proper medication were given placebos. Adams was receiving almost £1,000 for each patient.

Last month Adams was found guilty by the General Medical Council (GMC) of serious professional misconduct and suspended for 12 months. The GMC's lawyer described it as 'assault' and Sudano is now looking to sue Adams.

GlaxoSmithKline was using Adams to study its anti-depressant Paroxetine, but had to cancel the trial at a cost of hundreds of thousands of pounds.

The drugs industry talks of bad apples and the odd errant doctor milking the system but insists that trials using GPs are essential for medical advances and that payment to doctors for the extra work involved is ethically correct.

But an investigation by The Observer suggests the problem of GPs using patients as guinea pigs without their consent is more widespread. Some 3,000 doctors each year are paid by drug firms to sign up their patients to tests and on average 15 patients are needed for each trial. With doctors picking up £1,000 per patient, drug companies are spending £45m on getting doctors onside. There is no suggestion the companies are implicated in moves to carry out the tests by doctors on patients secretly.

Medical fraud experts estimate that one per cent of all drug trials involve fraud, including failure to get proper consent from patients. This means hundreds of patients a year are being given unapproved and potentially dangerous drugs without their knowledge.

The Observer has discovered that in the last five years the GMC has taken action in a dozen cases involving GPs undertaking fraudulent research. Examples include:

· Dr Vasu Agrawal from Chigwell, Essex, removed samples of womb from menopausal women as part of a trial into a new hormone replacement drug called Divina Nova. Agrawal failed to tell the women they were testing a drug that could have serious side-effects. Agrawal, who forged signatures on consent forms, received almost £6,000 from Orion Pharma International.

· Dr Paul Chima from Edinburgh is estimated to have received more than £200,000 from a range of pharmaceutical companies for testing their new drugs for angina, asthma, high blood pressure and depression. He failed to warn patients of possible side-effects and offered one a £2,000 bribe not to give evidence against him.

· Dr James Boschler from south London was given £22,500 from Bayer and Solvay Healthcare. He claimed to have signed up 36 patients, but 25 of the consent forms were discovered to be forgeries.

The country's foremost investigator of medical research fraud is Peter Jay, the former Metropolitan Police detective chief inspector who arrested serial killer Dennis Nilsen. Since 1996 Jay has run MedicoLegal Investigations, an independent body, and has taken 12 doctors to the GMC.

Jay is investigating six further cases, including one involving a GP in Manchester. Dr Mark Northfield is alleged not to have obtained consent from patients for entering them into trials to test drugs manufactured by Bayer and Roche to treat high blood pressure and heart problems. Northfield is contesting the allegations before the GMC professional conduct committee.

Jay said: 'While the industry has become more alert to the problems over the last decade there is still clearly a worrying and persistent problem in research fraud involving doctors. This is not just an issue of patient safety, but a problem that might lead to bad drugs being approved or good drugs failing to be approved.'

The Observer's revelations of the continuing problem of research fraud have sparked calls for industry guidelines to be toughened up. David Hinchcliffe, the Labour chair of the House of Commons Health Select Committee, described the situation as bordering on 'scandalous' and said his committee would look into the issue.

He said: 'The relationship between the drug firms and the medical profession is one that needs to be thoroughly investigated. It is extremely worrying that patients' trust is being abused by doctors who are more interested in making money from the pharmaceutical industry.'

Dr Evan Harris, health spokesman for the Liberal Democrats, suggested that doctors involved in trials should be subject to on-the-spot inspections and that a patient's consent to take part in a drug trial should be given outside the GP's surgery, for example to a research officer acting for the firm.

Additional research by Charlotte Coulon