A young TikTok user has long, wavy hair, glowing makeup and a radiant smile. She’s slim and wants you to know exactly why: she’s using Wegovy, a prescription drug originally developed to treat diabetes that’s become a popular drug for weight loss.

In one clip, she picks up the medication from a pharmacy, lip-syncing to Cardi B, then demonstrates in a following clip how she injects it into her leg. A caption flashes across the screen: “I’m not a doctor just FYI.” Moments later she advises her nearly 20,000 followers on how to get started on the drug. “Start on the 0.25 mg,” she says directly into camera. “Work your way up with each dose. Do not skip doses. I do not want any of you feeling sick.”

She’s what’s called a patient influencer. They have no medical training and claim that they’re simply sharing their personal experiences with their TikTok and Instagram followers. But in this quickly growing and largely unregulated arena, it’s gotten harder to tell when influencing crosses legal and ethical lines.

Many patient influencers offer prescription drug advice to their followers without always revealing their relationships with drug companies, according to Erin Willis, a University of Colorado, Boulder, associate professor who authored a study about patient influencers released this week.

Since patient influencers often share highly personal, vulnerable stories about their own health conditions, audiences find them trustworthy: a 2020 survey by a Wego, a major patient influencer agency, found that 51% of respondents said they mostly or completely trusted patient influencers, compared with just 14% who said the same for lifestyle influencers. An accompanying blogpost explains: “These patient leaders have built a well-established foundation of authenticity and trust in their communities.”

In exchange for hawking a health product or service, a patient influencer can expect to earn anywhere from “the low hundreds to a few thousand dollars” per social media post, depending on the health condition and the size of their online following, according to Amrita Bhowmick, the chief community officer at Health Union, a marketing firm that bought Wego in 2021.

Patient influencers can do this thanks to some of the world’s most permissive laws on prescription drug marketing. The US is one of two countries (the other is New Zealand) that allow direct-to-consumer (DTC) ads for prescription drugs. Since 1997, the Food and Drug Administration has allowed drug companies to push prescription medications on American airwaves as long as the ads are truthful, explain what the drug has been approved to treat, mention its major risks, and contain a disclaimer like “talk to your doctor”. Studies find DTC ads lead to doctors prescribing them more – driving the market for these ads to nearly $7bn last year, industry statistics show.

There are no published figures on the size of the patient influencer industry – but all indications are that it’s booming, says Willis. Medical ad agencies are typically tight-lipped about using patient influencers, but “they’re all engaged in this practice … this is a strategy that the pharmaceutical companies have found that works,” she says. Last summer, Willis spoke at a pharmaceutical marketing conference and asked the audience to raise their hands if they used patient influencers – nearly the whole room did.

Part of what makes patient influencers effective is that they often push messaging further than what would be allowed on media like TV, where ads are far more closely scrutinized by regulators like the FDA and Federal Trade Commission. Willis calls patient influencing “an interactive form of advertising” that’s “difficult to regulate, if it’s been regulated at all”. (In an emailed statement, an FDA spokesperson said the agency “takes its responsibilities seriously and will continue to monitor promotions and communications regarding prescription drugs through its surveillance operations, which include online platforms”.)

Willis found that all 26 patient influencers she spoke to in her study viewed themselves as “experts” and framed their efforts as raising awareness by sharing their own experiences. But some said they had discussed medications beyond those that they had taken, and many said they had discussed medications with followers over private messages. It’s those less visible kinds of content – including short-form and disappearing video – that are particularly concerning to Willis: “We don’t actually really know what all patients are doing, or what content they’re posting, or if they’re disclosing their relationship with pharmaceutical companies.”

In an email to the Guardian, Health Union’s Bhowmick says the company recruits and approves its influencers, whom it calls “patient leaders”, based on their “existing online presence”, or their participation on message boards that Health Union has set up for specific medical conditions, such as Migraine.com. Bhowmick, who helped Willis analyze her findings, says the company shares Willis’s concerns and “works with all our patient leaders to ensure they follow our best practices and community rules in all online activities – such as not providing medical advice and adhering to FTC guidelines for sponsored activities”.

Right now, we can only take their word for it. While federal law requires pharmaceutical companies to disclose the amount of money they pay doctors, no such rule exists for patients. And if an influencer doesn’t reveal that they’re on a drug company’s payroll, there’s no way to tell if it’s an advertisement that should be subject to regulation.

On a recent TikTok post by the young Wegovy influencer, one of her followers commented: “I’ve been on it for 3 weeks and haven’t lost a single pound. Pls tell me it will start to work.”

The influencer responded: “Bump up the dose.” Another commenter complains: “Made me soooo sick. Projectile vomiting because I didn’t poop for 10-15 days at a time.” The TikToker replies: “Stoppp!!! Omg!! Did you do the .25 dose??” With no listed sponsorships on her profile, it’s not clear whether she’s broken any advertising rules.

And with the countless patient influencers out there – Health Union alone boasts a network of “over 100,000 patient leaders” – it’s hard to say how many social media users are dispensing unvetted medical advice, or making money while doing so.

A big problem, Willis says, is there remains an “alarming lack of research” on the industry – and that research is difficult because “no one’s willing to talk about it”.

“When you ask advertising professionals, they’re not going to tell you much because of NDAs that they sign,” she says. And in her most recent study, “none of the influencers I spoke to were going to get into the weeds of the relationship [with companies] or the contracts with me. My thought is, if nothing is wrong about the practice, then why isn’t more known about it?”

• This article was amended on 17 March 2023. An earlier version incorrectly attributed the assertion that influencers may not reveal their ties to drug companies to the study rather than to Willis’s own observations.