A customer smokes a vape pipe in January at a shop in Richmond. (Steve Helber/AP)

Federal regulators issued warnings to four e-cigarette companies for failing to follow advertising rules for tobacco on their “influencer” social media posts.

The influencer endorsements on Instagram, Facebook and Twitter neglected to include the required nicotine warning, the Food and Drug Administration and the Federal Trade Commission announced Friday in a joint statement. The posts invited followers to try the liquid capsules, which come in an array of flavors, used in vape pens. The FTC, which polices unfair and deceptive corporate practices, said promotions lacking health-risk disclosures could be in violation of federal law.

The agencies sent warning letters to Solace Technologies, Hype City Vapors, Humble Juice Co. and Artist Liquid Labs. The companies did not immediately respond to requests for comment.

“These letters are a reminder that companies who use social media influencers to promote their products must comply with all applicable advertising requirements,” Andrew Smith, director of the FTC’s Bureau of Consumer Protection, said in a news release. “Moreover, ads must disclose material health or safety risks — in this case, the fact that nicotine is highly addictive.”

The warning letters include links and images of the influencers’ posts, which encourage their followers to buy vaping products in flavors such as peanut butter, mango and blueberry. The companies now have 15 days to take action and notify the FTC of their progress, the agency said.

The FTC also told the companies that influencers “should clearly and conspicuously disclose their relationships to the brands when promoting or endorsing products through social media.” The reminder highlights the agency’s ongoing effort to protect consumers who may not realize that online celebrities are paid to tout products in ways that don’t resemble a traditional advertisement.

The marketing practices of the e-liquid industry has attracted the scrutiny of the FDA and FTC before. Last year, the agencies issued 13 warning letters to manufacturers, distributors and retailers of e-liquids in packaging that appeared to target children, similar to packaging for juice boxes, candies and cookies, the regulators said.

In March, the FDA issued a new policy meant to combat what its former commissioner called “an epidemic” of youth vaping. The initiative limited the sale of fruity and “kid-friendly” vaping products to stores that have an adult-only section or that prohibit minors. It also compelled online vendors to upgrade their age-verification systems and limit bulk orders.

E-cigarette advocates contend that these products are mostly used by adults who are trying to quit smoking and view them as a less harmful alternative. But anti-tobacco and youth advocates say such claims are not supported by data, and they point to the rise in child vaping as a developing health crisis.

Juul Labs, the dominant player in the youth vaping market, stopped selling most of its flavored e-liquids in brick-and-mortar retailers last year and bolstered its age-verification process for online orders. The FDA policy also requires e-cigarette companies to apply, by 2021, for federal approval of their products to remain in the market.