The manufacturer of popular drugs Wegovy and Ozempic warned Thursday that it has seen a surge of counterfeit versions of its products in recent days.

Novo Nordisk said it "has seen a significant increase in illegal online sales, "according to a statement, referring to drugs that contain the active ingredient semaglutide. “Every counterfeit case we are made aware of is looked into and reported to the authorities according to the applicable regulations.”

The Denmark-based pharmaceutical firm said that in several cases of counterfeit drugs, it was working with “specialized firms to identify the perpetrators of these crimes." It did not provide details on the countries where it was working with local officials.

It also said it was working with a third party specialized in monitoring and taking down illegal online offers. Novo scaled up this work in 2023 “to ensure an even broader reach," it said, without naming the company it was working with.

The statement from Novo, earlier reported by Danish publication Finans, included the first detailed comments from the group on the counterfeit issue since a statement in mid-June about fake Ozempic pens found at a retail pharmacy in the United States.

In a further sign of illicit activity taking advantage of the hype around weight-loss drugs, the U.S. Food and Drug Administration (FDA) on Tuesday told two online vendors to stop selling unapproved versions of Novo and Eli Lilly drugs.

The FDA this week sent warning letters to two companies, Semaspace and Gorilla Healing, to halt selling online versions of semaglutide and tirzepatide.

"As discussed below, [the] FDA has observed that www.semaspace.com introduces into interstate commerce unapproved and misbranded semaglutide drug products. There are inherent risks to consumers who purchase unapproved new drugs and misbranded drugs," one of the letters stated. "Unapproved new drugs do not carry the same assurances of safety and effectiveness as those drugs subject to FDA oversight."

In its letter to Gorilla Healing, the FDA said it "recently reviewed your website at the Internet address www.gorillahealing.com and has observed that your website introduces into interstate commerce misbranded and unapproved new drugs in violation" of federal law.

The FDA said it is demanding that the two firms “cease offering any unapproved and misbranded drugs for sale to U.S. consumers,” claiming that the move is “critical to protect the public from harm.”

The two firms have until the end of October to provide an explanation. But if they cannot, the FDA can take legal action against them, including seizure or an injunction.

In recent months, demand for Wegovy, Ozempic, and semaglutide products has surged in the United States and other Western countries amid a years-long obesity crisis. Novo was recently forced to restrict doses of Wegovy to the U.S. market due to potential supply concerns.

FDA Warnings

In late September, the FDA announced that the drugs can lead to a potentially life-threatening condition known as "ileus," in which the intestine can become blocked.

“The following adverse reactions have been reported during post-approval use of semaglutide, the active ingredient of OZEMPIC. Because these reactions are reported voluntarily from a population of uncertain size, it is not always possible to reliably estimate their frequency or establish a causal relationship to drug exposure,” the FDA noted on its website. It now lists ileus as a possible adverse reaction.

Ozempic works by mimicking the hormone GLP-1, which can slow down the passage of food in the large and small intestines as well as the stomach, making people feel full for longer. However, there have been reports that the drug can slow the intestines too much, leading to ileus, causing intestinal blockage.

Earlier this year, the FDA warned consumers not to use off-brand versions of Ozempic, Rybelsus, and Wegovy because they may not have the same ingredients.

Those off-brand versions of the drugs are possibly unsafe or ineffective, the federal regulator said in a notice this week. Officials said they received reports of problems linked to “compounded” versions of semaglutide, the drug’s active ingredient.

“Drug compounding is the process of combining, mixing, or altering ingredients to create a medication tailored to the needs of an individual patient,” the agency said in June. “Compounding includes the combining of two or more drugs. Compounded drugs are not FDA-approved, and the agency does not verify the safety or effectiveness of compounded drugs.”

Compounded semaglutide can contain a version of the ingredient that is not approved for human use, said the FDA. It also warned that reports have indicated some versions of compounded semaglutide contain salt, which changes the drug.

“The agency is not aware of any basis for compounding using the salt forms that would meet the [Federal Food Drug and Cosmetic Act] requirements for types of active ingredients that can be compounded,” the FDA said.

Reuters contributed to this report.