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If you’ve spent any time on Facebook in the past few years, there’s a good chance you’ve seen someone you know selling the skin care brand Rodan + Fields. The company is a multilevel marketer, which means its main sales strategy involves employing an army of 200,000 consultants, some of whom you probably went to high school with. Products are also sold on the brand’s site. You might want to pause before letting her convince you to buy the brand’s eyelash serum, Lash Boost, though.

Rodan + Fields was just slapped with a potential class-action lawsuit. The suit was filed on April 13 on behalf of four plaintiffs, two in New York and two in California. It takes aim specifically at Lash Boost, an eyelash growth serum the company launched in 2016.

The product has been a hit for the brand; a company representative told WWD it did more than $200 million in sales its first year. Lash Boost is marketed as an “eyelash-enhancing conditioner” and claims to provide “the appearance of lush, longer-looking lashes in as little as four weeks.” It costs $150 for a tube.

All those tubes have certainly helped the company’s bottom line. Rodan + Fields just announced that it is now the No. 1 beauty brand in both the US and North America, according to Euromonitor, a market research provider. (Neutrogena and Olay are in second and third place, respectively.) The company claims it hit $1.5 billion in sales in 2017, up from $1.15 billion in 2016, according to WWD.

Racked obtained a copy of the complaint from attorneys at Keller Rohrback LLP, who are representing the plaintiffs and have requested the court certify the complaint as a class action. It alleges that the four plaintiffs all suffered adverse eye reactions after using the product, which is meant to be applied to the upper eyelids.

They said they experienced symptoms including bumps on the eyes, flaky patches, burning, swelling, crusting, and pain, among other things. The complaint alleges “deceptive labeling and unlawful marketing” of Lash Boost and that Rodan + Fields “failed to disclose the harmful side effects linked to an ingredient in their Lash Boost product.”

A representative for Rodan + Fields provided the following statement to Racked:

The Company vigorously denies the allegations in the Complaint, and stands behind the safety and efficacy of Lash Boost. We are going to let the specifics of our legal defense play out in court. Lash Boost is intended for use as a cosmetic and as such, has been consistently advertised as improving the appearance of eyelashes. As with any cosmetic, Lash Boost may cause irritation in some users, especially if it is misused. R+F provides clear directions to users, including those who experience irritations. Many of the allegations involve unrelated products, including prescription products that have different ingredients and formulations.

The ingredient in question is isopropyl cloprostenate, which is commonly used in lash growth products. It has a controversial history, both legally and in terms of its alleged side effects.

Before you understand Lash Boost, though, you have to understand the OG lash growth product, Latisse. Latisse was released in 2008 as a prescription-only eyelash growth serum after Allergan, its parent company, discovered that a medicine it was using for glaucoma also gave patients long, lush lashes.

It then developed the serum as a topical drug specifically for eyelash growth, which was approved by the Food and Drug Administration. The active ingredient is bimatoprost, a prostaglandin analog. (A prostaglandin is a chemical in the body made of fatty acids that has hormone-like effects; an analog is a synthetic version that has similar qualities to the original.)

Latisse was a hit, and celebs like Brooke Shields and Claire Danes shilled it. It became clear, though, that Latisse had some worrisome side effects, including causing eyelid discoloration and possibly even changing iris color permanently.

In 2009, the FDA issued a warning to Latisse that some claims were misleading and safety warnings were not adequately stated, which the company later rectified. There were even rumors back in 2010 that Danes actually experienced some of these side effects.

A post shared by Rodan + Fields (Official) (@rodanfields) on Aug 1, 2017 at 7:28am PDT

Then the copycats came. Allergan ended up suing several companies that were selling products as over-the-counter cosmetics that contained prostaglandin analogs. It won on the basis that they were unfair competitors. But these types of products have been popular ever since; you can find several at Sephora, including some that contain isopropyl cloprostenate, the prostaglandin analog at the center of this new Rodan + Fields lawsuit.

Isopropyl cloprostenate has never been approved by the FDA as a drug. Because of the lax cosmetics oversight regulations in the US, brands can sell it as a cosmetic product, bypassing the rigorous testing that prescription drugs are subjected to. (It’s banned in Canada, which is why Rodan + Fields can’t sell Lash Boost there.)

In 2011, the FDA issued a warning letter to the company that makes the lash serums RapidLash and Neulash. Based on the labeling the company was using at the time and the known side effects of prostaglandin analogs, the agency called the products “unapproved” and “misbranded” drugs.

The FDA also noted that as prostaglandin analogs, they carried the risk of serious side effects like irritation, iris color change, inflammation, and eye pressure changes. RapidLash has been reformulated, but it appears that Neulash still contains isopropyl cloprostenate. The FDA has not appeared to issue this type of warning to any eyelash serum manufacturer since then.

This brings us back to Rodan + Fields. The FDA is not involved in this situation at all. The lawsuit does reference the 2011 FDA warning letter in the complaint against the company, though, and alleges that Rodan + Fields “failed to disclose the harmful side effects linked to an ingredient in their Lash Boost product.”

Lash Boost notes on its warnings page: “For external use only. Avoid getting in the eye; in the event of direct contact rinse with cool water. If you develop irritation or swelling, discontinue product usage. If irritation is significant or in the first instance of any swelling, consult your physician. If you’re pregnant or nursing, being treated for any eye-related disorder, undergoing cancer treatment, prone to dry eyes or styes, consult your physician before use. If you notice irregularities in appearance of lashes, discontinue use. Keep out of reach of children.”

Wording definitely matters, both in the warnings section and in descriptions of product claims, and Rodan + Fields addresses that on its website. Part of the reason those older companies attracted the attention of the FDA is that they made claims that their product can actually grow lashes. Lash Boost is careful to say things like “improves the appearance of lash volume and length.” By focusing on appearance and making no specific claims about changing function or structure of the lashes, Lash Boost seems to adhere to the letter of the law for cosmetic versus drug product labeling.

As far as side effects, this is not the first time users have reportedly experienced unpleasant reactions from Lash Boost — product users online and on YouTube allege various side effects. Obviously, the plaintiffs (and any others that come on board if this lawsuit gets certified as a class action) will have to convince the court their arguments have merit.

Isopropyl cloprostanate is not technically a drug, but should it be? That’s the crux of this lawsuit. It’s a complex issue that the courts and ultimately the FDA will have to address.