The following correction was printed in the Guardian's Corrections and clarifications column, Friday April 28 2006.

The article below states that the Muscular Dystrophy Campaign is one of six patient organisations that do not receive funds from drug companies. The campaign has asked us to point out that it is supported by GlaxoSmithKline and that the error occurs in a report for a parliamentary committee, which was the source of our information.

Lisa Jardine was at home recovering from chemotherapy one evening last May when the phone rang. She was not feeling all that well; the conversation that followed made her feel worse. There was only one breast cancer story around last May, as is still true today: Herceptin. It was a wonder drug - it halved some women's chances of having a recurrence of their cancer. But women who would die without it were being denied access, apparently for financial reasons - or so the story went. Women with aggressive, early-stage breast cancers had taken to the streets and the courts for their right to get it. So when - a week before the phone call - Jardine, who had had breast cancer, was asked in a newspaper interview what she thought about Herceptin, she responded that although she was confident she was receiving the best of care, "if Herceptin really is as effective as we are being told, I do feel I ought to be given the choice".

Then came the phone call. "Halfway through the following week, the phone goes at home," says Jardine, professor of Renaissance studies at Queen Mary, University of London, writer and well-known television presenter. "It's a really nice woman. She says to me, 'I read about you in the paper and I gather you'd like access to Herceptin and you can't get it.'"

By now, however, Jardine had decided that she did not want the drug. "I said, 'No - that's not the case with me. I have decided not to have Herceptin.'

"She said, 'Even if you don't want it yourself, would you come and talk to some of our seminars because we're running a big campaign to promote Herceptin? Either we could find funding for Herceptin or, if you really don't want it or decide against it, there would be fees for appearances.'

"I said, 'Could you tell me where you are from?' She said, 'We work for Roche.'

"I wasn't feeling well. I said, 'Would you please get off the phone?' Then I hung up.

"There was no mistaking the directness of the approach - she said she would make it worth my while."

Jardine was shocked that a drug company, or a PR agency working for one, might attempt to use her as part of its marketing strategy. But the fact is that this sort of approach is exactly what pharmaceutical marketing is all about in an era when the stakes for drug companies seeking to make profits are climbing steadily higher. Jardine's experience only serves to throw a little light on the increasingly sophisticated way in which new drugs are sold to us in this country these days - generally without us realising that a marketing operation is even under way. It helps to explain how a drug such as Herceptin, despite being as yet unlicensed for use on women with early-stage cancer, and despite there being only a few years of test data from its manufacturers Roche to support claims being made by some for benefits for this group of women, came to be a household name and a cause celebre. At the time that Jardine got that phone call, health secretary Patricia Hewitt, under huge pressure, had already appeared to cave in to demands for it to be prescribed more widely - she had publicly declared that cost alone should not prevent women from being given it. It was a green light to doctors to prescribe and to primary care trusts to pay up, even though the National Institute for Clinical Excellence (Nice) had not yet approved the drug for use on women with early-stage cancer.

Herceptin is a new type of drug, suitable only for 20% of breast cancer patients. Roche, naturally, having spent many millions of pounds on developing the drug, badly wants as big a slice of this restricted market as it can get. But, in this country at least, it cannot buy television advertising. Drug companies, unlike any other industry, are forbidden to promote their goods direct to you and me. So instead, Roche has found itself some powerful allies. It cannot sing the praises of its latest drug from the rooftops and demand that doctors hand it out - but patients can, and will, too, if they become convinced that this is the pill they need.

Just a few years ago, nobody had heard of Herceptin, although stories about a new generation of "targeted drugs" had begun to circulate, raising hopes of medicines with better cancer-killing powers. In 2002, Nice licensed Herceptin for use on women with advanced breast cancer; it undoubtedly saved lives. But then Roche did trials on women who had just finished chemotherapy for early-stage cancer. The company had not yet applied for a licence for this change of use in this country, but it decided to announce interim results showing that the drug halved the risk of the cancer recurring. The clamour for Herceptin began, and amid all the noise, any chance of a reasoned debate about the drug was lost.

Unlike many in the UK, Jardine knew that the drug was not problem-free. Some women in the US had suffered from heart problems after taking it. That was why Jardine and her consultant had decided the drug was not for her.

Roche argues that the cardiotoxicity - which led to 18.6% of the women in one US trial stopping the drug within a year - was down to the other chemotherapy drugs not used here which are taken routinely by women in the US. It says the damage is reversible and that doctors will monitor patients closely for it. Only 0.5% in British trials had heart problems on the drug, they say. That translates as nine patients developing severe congestive heart failure out of a group of 1,694 women who had been carefully scrutinised and monitored for potential heart problems before they were enrolled on to the trial. A larger 1.7% developed warning symptoms of heart failure during the trial, and a total of 8.5% stopped taking the drug before the year was up either because of side effects or other reasons.

But the heart problem is not the only issue. Critics argue that, because Roche's trials of Herceptin have been stopped early - on the grounds that early results were so good that it had become unethical not to give the drug to women on the trial being given a placebo - and with only a year of data in existence for Herceptin used alone (although one of the trials is following up patients to two years), the best that can yet be said is that the drug lengthens the interval before cancer recurs.

The statistic that keeps being repeated is that Herceptin halves a woman's chances of getting a cancer again. But the bottom line on this drug is that the 50% reduction in cancer recurrence is a relative risk. It actually means that 9.4% of the women in the British trial on Herceptin found their cancer returned, compared to 17.2% among those who did not have the drug. It is a good result, but it does not have quite the simplistic punch of "halving the rate of recurrence". And we do not know if the benefit lasts.

We cannot possibly know yet, from the data available, whether the drug stops people dying - and all the "Last chance for breast cancer victim" headlines (as in yesterday's Sun) do not change that. The published results spell it out clearly in the New England Journal of Medicine: "Overall survival in the two groups was not significantly different." So why are women talking of selling their homes for a "life-saving drug" and going to court for a "lifeline"? (Just yesterday, Ann Marie Rogers was at the high court in a final attempt to force her health trust to pay for Herceptin - something it declines to do while Nice has still not pronounced on the drug.)

Dot Griffiths has some of the answers. This is the formidable woman behind the successful struggle in Staffordshire to get Herceptin funded on the NHS for eight women with early breast cancer that turned into a nationwide campaign for "all women who need it". But Griffiths has advanced breast cancer - not early breast cancer. More than four years ago, Herceptin was only available privately even for advanced cancers and she successfully fought her own campaign to get it. Her clinicians thought she would be dead by now, she says. That makes it a wonder drug in her book.

"I had never campaigned before in my life. I have always believed in fair play and justice. I was a magistrate for 16 years - I hate injustice of any kind," she says. "All that I say to everyone is that I'm living proof that it is as good as I think it is. I'm still here four and a half to five years after having it, with a very good quality of life."

When, in March 2002, Nice approved Herceptin for general NHS use for advanced breast cancer like Griffiths', that meant all primary care trusts should pay the £30,000-a-year cost if a doctor wanted to give it to his or her patient. Roche, naturally, hoped that there would be a big take-up, but it was not as fast as it could have hoped. The following year, the drug company carried out a survey to see how many of the women who were suitable were getting Herceptin. Its figures suggested only a third were being given the drug.

Roche gave the survey to the patient group CancerBacup, which publicised the results widely. Its chief executive, Joanne Rule, went on the BBC's Breakfast Time programme. Naturally, the show wanted a patient, too. Roche's PR company contacted Griffiths.

"There was a PR agency called Ketchum which worked for Roche," she says. "They put me in touch with CancerBacup. I'm very grateful to Roche, but I wasn't campaigning for Roche. I was campaigning for women like myself who need the drug."

Griffiths' campaign moved on from the women with advanced breast cancer like herself to the women with early breast cancer who might benefit from Herceptin. She hoped the drug she believed had saved her life would now save the life of these other women. She is insistent that the drug company did not support her in all this. "They didn't help me at all. I have had contact with Roche, naturally, because I wanted facts and figures about the drug and I have done a presentation for breast cancer awareness month at Roche, but [I didn't] mention the campaign at all, because they want to keep well away from [it]. They don't want any involvement with the campaign."

Clearly, Roche did not need to help Griffiths. But the PR agencies which bid for lucrative contracts from the drug companies would understandably like to be associated with such an emotive and hugely successful campaign. Rebecca Hunt, formerly a Roche employee but now with the agency Porter Novelli, offered the services of her team for free.

"I met Rebecca Hunt when she worked for Roche," says Griffiths. "They are a lovely bunch of people at Roche. She rang me about two days before the march was taking place."

Griffiths was working so hard that she put herself in hospital for several weeks. She gladly accepted Hunt's offer to put out a press release about women marching on Downing Street over the funding of Herceptin. "Women protest to save 'lost generation' of breast cancer sufferers," it ran.

"We gave our time for free," says Hunt. "It was pro bono. It was really inspiring to the team here to get involved in something that makes so much difference to people."

"She's so nice," says Griffiths of Hunt. "She helped us out on that occasion. I don't know if she did anything else."

Griffiths' campaign is a one-off. Pharmaceuticals do not usually get that lucky. It is far more normal for the companies to offer support to large, established patient support groups.

Breast cancer charities have been vocal in support of wider availability for Herceptin for women with early breast cancer - and they have been vocal long before Roche got around to applying for a UK licence, a process that involves scrutiny of its data by experts. The science-based charity Cancer Research UK has stood out in its avoidance of hyperbole. "The results of these trials seem to be quite startling," says its website. "But do remember, these are very early results."

CancerBacup has led the charge against "postcode prescribing" - where some health trusts will pay but others won't - for a number of drugs. "Even though Herceptin doesn't yet have a licence for early breast cancer, all good private medical insurers are already funding this treatment because of the strength of the clinical trial data. The time to act is now," Rule said last October.

CancerBacup's latest financial statement showed that it received 31% of its income from companies and charitable trusts. They include Roche and around 14 other major drug companies. That doesn't matter, says the charity, because it has guidelines on relations with the industry. While it must not compromise its integrity or damage its reputation, "we believe it is important to maintain cooperative relationships with companies that manufacture and market cancer drugs and other treatments, in order to foster communication between the patients CancerBacup represents and the companies whose decisions will affect their treatment".

It is not just the patient groups that drug companies hope to get support from. They also want "opinion leaders" - people with credibility who can be quoted in the papers and on TV - people such as Jardine. And they want journalists. A fortnight ago, Roche held a think tank for journalists from national newspapers, magazines and television. The journalists were asked for their opinions on how best the company could get stories into the media about its drug for breast cancers that have spread into the bones. They were promised £250 each for their time and given dinner at an expensive restaurant.

But patient groups are the most rewarding target and there is an obvious risk that they could be influenced by companies with turnovers as large as the GDP of small nations. The industry journal Pharmaceutical Marketing ran a recent piece describing how "motivated patients can move mountains and boost your drug's fortunes".

Labour MP Paul Flynn has done a survey to find out which patient groups accept drug-company money. Out of 24 major organisations, only six did not. They were Alcohol Concern, Action for ME, Muscular Dystrophy Campaign, Young Minds, Mind and the National Autistic Society.

Besides the Herceptin bandwagon, there are a number of other drug bandwagons trundling along, making the case for wider or immediate availability of drugs that may not have been licensed or approved by Nice. It puts pressure on ministers and, in the end, must surely undermine Nice. That might not matter if all the evidence were as clear-cut as has been suggested and all campaigns were entirely transparent. But it is not so.

From April, under new industry rules, the drug companies will have to declare on their websites what money they give to patient groups. It should make interesting reading.