Kathleen Turner was on television recently talking about her pain and suffering. "The damage that I have, the damage I'll always have could have been prevented," the actress told Diane Sawyer, the host of Good Morning, America. Sawyer was sympathetic. Turner, she knew from a previous interview, had been battling rheumatoid arthritis for more than a year.

"You're still in pain?" Sawyer asked. "Well," Turner responded, "as they say: only when you walk."

Turner then went on to mention a website, www.ra-access.com, where fellow sufferers could get help. Sawyer eagerly repeated the site's address in case viewers missed it.

All of this would have been your standard bit of talk-show chitchat if Turner had not been paid by two drug companies to speak out about her illness. "She gets a fee," confirms Robin Shapiro, a spokeswoman for Immunex, a bio-pharmaceutical company, which along with fellow pharmaceutical giant Wyeth, funded a media campaign for which Turner was hired to do a number of TV and print interviews.

The two companies jointly manufacture an arthritis drug called Enbrel. They also happen to operate the website that Turner and Sawyer plugged as so helpful to arthritis sufferers. The site is a marketing tool for Immunex and Wyeth. Visitors are asked to supply personal information, which, according to Shapiro, is used to send them promotional materials on Enbrel, a drug that competes with many others in the multi-billion-dollar market for the treatment of rheumatoid arthritis.

This kind of below-the-radar media campaign, which blurs the line between drug advertising and public service efforts, is now rampant. And increasingly crucial to the success of these carefully orchestrated blitzes are celebrities willing - for a price - to pour their hearts out about how they or a close relative have struggled with a particular illness. West Wing star Rob Lowe, for example, recently embarked on a drug company-sponsored awareness campaign for a chemotherapy-related illness called febrile neutropenia (which his father suffered from). The work will reportedly net Lowe $1m.

"It has become a hugely grey area as to where the stars' charitableness begins and where their financial interests begin," says Barry Greenberg, a Hollywood talent agent whose firm, Celebrity Connection, specialises in recruiting stars for the drug companies. "Maybe Katie Couric wasn't being paid to have a hose shoved up her ass [the co-presenter of NBC's Today programme was filmed having a colonoscopy as part of a feature on colon cancer], but the rest of the stars, they're getting something."

In 1997, under growing pressure from pharmaceutical companies, the US Food and Drug Administration relaxed its regulations on television advertising for prescription drugs. The industry promptly went on a multi-billion-dollar spending spree, bombarding the airwaves with commercials for their products, and pharmaceutical PRs made immediate efforts to tap into star power. Joan Lunden was hired to hawk the asthma drug Claritin in 1998. Bob Dole promoted Viagra in 1999.

These first ads were like most others that used celebrity endorsements: the star's connection to the company was obvious; the pitch was straightforward promotion. But, as one marketer for the drug industry puts it, "There was a backlash." It occurred to consumers and advocacy groups that it could be dangerous for trusted public figures to recommend drugs with serious side effects. In some cases, celebrities with transparent links to drug companies saw their reputations seriously tarnished.

So the drug companies tried a different tack - celebrity-driven public-awareness campaigns that obscured the financial relationship between the star and the drug company, while allowing both parties to avoid any talk of side effects or potential problems with the drug. With all hints of drugs and money hidden from view, celebrities were again willing to sign up, taking fees of as much as $1m to do a raft of television and newspaper interviews in which they spoke about a particular illness and urged sufferers to seek treatment.

For the most part, the celebrities don't mention their sponsor or its product by name, but instead urge people to go and see their doctors about the latest treatments, or, in the case of Turner, suggest that viewers or readers visit a specific website.

This approach allows the drug companies and their celebrity hirelings to insist that they're doing a public service, not to mention providing the public with compelling human-interest stories and sneak peeks into the private lives of the stars.

These awareness campaigns also happen to be a way for the drug companies to avoid the remaining FDA regulations on TV advertising of prescription drugs. In a straightforward TV commercial for a drug, viewers must be told about the drug's major side effects or be directed to another source where they can get further information. Awareness campaigns are exempt from these restrictions because the FDA doesn't consider them advertisements. Thomas Abrams, who monitors marketing by the pharmaceutical industry for the FDA, says the agency would only take action against a drug company if the star employed by the company greatly exaggerated a drug's benefits. Otherwise celebrities, like any other citizen energised by a cause, are free to say whatever they please.

On May 23 2000, actress Olympia Dukakis appeared on the Fox News channel to talk about her mother's post-herpetic neuralgia (PHN), a debilitating condition brought on by shingles. Her mother "would sit muted in pain", Dukakis told viewers. "She couldn't even touch her hair. That's how painful it can get." Dukakis then went on to say that others need not suffer like her mother did, that there were new, better treatments out there. "In fact, there is one solution," she said. "It's called lidocaine or lidopatch."

PHN sufferers who went to their doctors and asked about lidocaine or lidopatch would have found out that there is only one drug Dukakis could have been speaking about, Lidoderm. Lidoderm is a patch which at present is the only formulation of lidocaine approved by the FDA to treat PHN. It also happens to be manufactured by Endo Pharmaceuticals, which was paying Dukakis to do the Fox interview. "Yes, she was compensated," says Sherri Michelstein, who was hired by Endo to organise the media campaign.

By mid-1999, pop singer Carnie Wilson weighed close to 300lbs and was, as she later told an interviewer, "desperate". She felt depressed, suffered from diabetes and a sleeping disorder, and was barely able to get her 5ft 3in frame in and out of her car. Her career had largely been in a downward spiral ever since the breakup of her rock group Wilson Phillips.

It happened that her former manager, Mickey Shapiro, had started a health-related internet company whose board of directors included surgeon Jonathan Sackier. He suggested that Wilson undergo gastric bypass surgery, which meant having her stomach stapled down to the size of a fist so that she couldn't eat so much. Wilson not only agreed, but volunteered to let Shapiro's website broadcast the procedure live.

Over the next three years, Wilson became the nation's poster child for the benefits of gastric bypass. "Since the surgery, I feel lighter, I feel sexier - inspired," a 150lb Wilson told People magazine (and many other media outlets) in April 2000. People did a total of three stories on Wilson, two of them front covers. She was also interviewed on ABC's Good Morning, America six times and on 20/20 five times. Asked once why she chose to go public about her obesity, she said: "If it can help people, then I'm happy to do it."

But it wasn't just about helping people. Wilson is paid to do appearances where she talks about her surgery. Her corporate sponsors are Tenet Healthcare, a for-profit hospital chain that does gastric bypass operations, and Vista Medical Technologies, which makes the medical equipment for the procedure.

Wilson doesn't see anything wrong with taking money from Tenet and Vista. "I don't need the money. That's not why I do it," she says. "Every day of my life I am committed to people and helping them get healthy." Wilson says she doesn't believe her corporate connections need to be revealed to the public. "That's all bullshit," she says. "We're talking about health issues and being able to help somebody by making them more informed."

There is no doubt that Wilson's outspokenness has played a huge role in popularising a risky, complicated and expensive procedure. The American Society for Bariatric Surgery credits her with playing a major role in publicising the benefits of gastric bypass, which has seen a huge surge in popularity since 1999. Vista has seen its revenues double. According to Vista's CEO, John Lyon, it makes little difference that Wilson doesn't mention Vista by name in any of her media interviews. "The rising tide lifts all boats," he says.

The public rarely learns if a celebrity is being paid for their endorsement. According to ABC spokeswoman Lisa Finkel, deadline pressure makes it hard to investigate a celebrity's hidden agenda. The result, she says, is that ABC did not know that Turner and Wilson had corporate backers. "It's troubling that certain of our guests have not disclosed their paid relationships with these companies," she says.

But it's not hard to find out who is really behind health awareness campaigns. A quick search of the internet turns up press releases put out by PR firms or drug companies announcing their affiliation with a particular celebrity. It seems less a matter of the media being fooled than of the media not really wanting to know. As one ABC News executive says: "The bottom line? If a celebrity has a compelling story to tell, we want to tell it."

· This is an edited version of an article that first appeared on Salon.com..