Top plastic surgeons and cosmetic dermatologists gathered at the Ritz-Carlton in Cancun one weekend this month to learn about a wrinkle-smoothing injection, Jeuveau, that goes on sale this week.

Jeuveau’s manufacturer, Evolus, billed the event as an advisory board meeting. But it also appeared to double as a lavish launch party for Jeuveau, which the company is hoping will compete against Botox in a crowded market that also includes two other products.

More than a dozen top doctors gushed about the event on social media — using the company’s preferred hashtag, #newtox — without disclosing that Evolus had paid for their trips. The Federal Trade Commission requires social media users to disclose relationships with companies when promoting their products on social media, which has emerged as a potent platform. Medical experts also said the tactics carried echoes of an earlier, anything-goes era of pharmaceutical marketing that the industry largely abandoned after a series of scandals and billion-dollar fines.

There was poolside socializing, free gifts and an oceanfront dance party, an atmosphere that one Manhattan plastic surgeon told her 187,000 followers was “everything Fyre Fest was supposed to be.”

“Everything is coming up BUTTERFLIES,” Dr. Melanie Petro, a plastic surgeon in Alabama, told her 74,000 followers, beside a photo of her posing on a runway that featured Evolus’s butterfly logo.

The Manhattan plastic surgeon, Dr. Lara Devgan, addressed her followers from the Ritz-Carlton’s oceanfront pool. “It’s the new tox on the block,” she said as she jiggled a pink Newtox bracelet for the camera. “I’m here making a splash with Newtox,” she said later.

Bonnie Patten, the executive director of Truth in Advertising, a nonprofit watchdog group, said that F.T.C. rules would most likely require disclosure of compensation for paid trips or hotel stays on social media posts promoting products. A spokesman for the F.T.C. said the commission did not typically comment on individual cases.

“It’s incredibly problematic, because health care providers in general are considered very trustworthy,” Ms. Patten said. “And especially by their followers, who are looking for their expertise.”

In an interview, Dr. Devgan said it was standard practice for pharmaceutical companies to cover her expenses to medical meetings, and she did not think it was necessary to disclose this. She said she would not favor Jeuveau over competitors, and noted that she said as much in her video. “I’m always trying, with my social media and traditional media presence, to be very neutral,” she said.

Dr. Petro said she traveled to Cancun to learn about the product and came away impressed. She said she didn’t think she was endorsing Jeuveau, but would consider disclosing to followers that Evolus had paid for her trip. “I never would want to be dishonest with them,” she said.

David Moatazedi, the Evolus chief executive, said in an interview that the Cancun event was a standard advisory board meeting, similar to those that the company’s competitors hold, and that the doctors were not paid or given incentives to promote the company. The company also noted that the doctors sometimes used the hashtags of competing products, like #botox, in their posts.

However, Mr. Moatazedi did say the company offered doctors what he called “social media moments,” like the Evolus-themed runway or a confetti-throwing station.

“We wanted to make the break periods of this meeting productive as well for doctors, and many of them like to inform their patients around the newest technologies,” Mr. Moatazedi said.

To Wall Street investors, Evolus has pitched its unconventional approach as a way to distinguish its product from Botox, made by Allergan, which commands about 70 percent of the more than $1 billion market for wrinkle-smoothing injections. Jeuveau, which was approved by the Food and Drug Administration in February, is Evolus’s only product.

The company is targeting the millennial selfie generation — which is increasingly interested in cosmetic procedures — with advertising featuring fresh color schemes and an emoji-filled social media campaign. Mr. Moatazedi has told Wall Street analysts that Evolus should be viewed as more of a “performance beauty” company rather than a traditional drug maker.

Botox and its competitors — including Dysport, sold by the Nestlé subsidiary Galderma, and Xeomin, sold by Merz — are used virtually interchangeably by doctors, who buy the products from the companies and charge patients for the procedures.

Mr. Moatazedi, told investors last November that his company did not have to report payments it makes to doctors to the federal Open Payments database, because Evolus does not sell any products that are reimbursed by government programs like Medicare or Medicaid. (Botox is also approved for migraines and other uses, which are covered by government health programs, while Jeuveau is not.)

“That means that the representatives and the sales managers can be very closely involved in high touch and customer-centric, and engage with these practices outside of their traditional business hours,” Mr. Moatazedi told Wall Street analysts during a conference call last November.

“They’re not hiding anything, not trying to subvert the law,” said Michael Moretti, the chief executive of Medical Insight, a market research firm for the aesthetics industry. “They’re just doing marketing in the way they can, and that their competitors cannot.”

Others said the company and doctors were treating the product too casually, especially since it is an F.D.A.-approved drug that carries a serious boxed warning. Like Botox, Jeuveau is a form of botulinum toxin and when injected, it can spread to other areas of the body, potentially causing swallowing and breathing problems in rare cases.

“I don’t think that it’s fair to call it a purely aesthetic agent,” said Dr. Aaron S. Kesselheim, a professor of medicine at Harvard Medical School. “It seems to diminish the risks to the product and overestimate or oversell the benefits.”

Drug companies do pay for doctors to attend advisory committee meetings and cover meals during educational events, but their practices are more limited than they once were, after several companies paid billion-dollar fines to settle charges they had inappropriately marketed their products.

Most companies now say they follow an industry code that prohibits extravagant trips, “recreation or entertainment” and the distribution of gifts for personal use. Any meals provided must be “modest.”

At the Cancun weekend, doctors posted photos of the Evolus-branded items they were given, including flip-flops, beach towels and water bottles.

They posted videos that offered a panoramic view of the thumping dance party that featured a giant video screen, neon water drums and fireworks. (Dr. Pantea Tamjidi, a Maryland dermatologist who posted a video of herself playing drums at the event, did not return a call and Instagram message for comment.)

Doctors who attended the event said that the company’s marketing would not affect their medical decisions and that they had been invited for a working weekend to provide expert advice.

“I use my knowledge and experience to research and evaluate a product and to determine whether it’s something I can use for my practice,” said Dr. Christopher Zoumalan, a Beverly Hills plastic surgeon who posted about the event to his 21,500 Instagram followers. “This has nothing to do with whether the company takes me to dinner or Cancun or whatnot.”

A dermatologist, Dr. Jeffrey Fromowitz of Boca Raton, Fla., said his post was not intended to be an endorsement. “I try to post variable topics of interest to our followers, which of course would include something new coming to market,” he said.

In clinical trials, Jeuveau performed as well as — but not better than — Botox. Still, some doctors described Jeuveau as “the happy toxin” on Instagram. One, Dr. Kaye Riolo of Fresno, Calif., used the hashtag #latestandgreatest to describe the product. She did not return calls and emails for comment.

The F.T.C. requires disclosure when a company pays someone or provides something for free — like a trip to Cancun. In 2017, the F.T.C. sent letters to 90 social media influencers and companies reminding them that they should “clearly and conspicuously” disclose relationships to sponsors.

Ms. Patten of the truth in advertising group said the positive posts about the product that lack disclosures about financial incentives like trips would be considered endorsements.

Promotion of prescription drugs carries an added layer of scrutiny because drug makers are required to balance a drug’s benefits with its risks in any advertising. In 2015, the F.D.A. sent a warning letter to the drug company Duchesnay after the celebrity Kim Kardashian posted on Instagram about its morning-sickness drug, Diclegis. Ms. Kardashian did not adequately describe the drug’s risks, the agency concluded.

Evolus said that it did not believe it was violating any F.T.C. rules and that doctors were compensated only for their expert medical advice.

Mr. Moatazedi said all promotion of Jeuveau included the required balance of risks and benefits, and said terms like “happy toxin” and “latest and greatest” were not part of Evolus’s marketing efforts. “That’s likely something the doctors are doing on their own,” he said.

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