Sunscreens come in many different forms, including lotions, creams, sticks, and sprays, to offer effective protection from the sun's harmful UV rays.  But one formulation consumers should steer clear of is so-called sunscreen pills, the U.S. Food and Drug Administration warns.

The FDA sent warning letters to companies illegally marketing and selling pills labeled as dietary supplements that make unproven claims about protecting consumers from the harms that come from sun exposure.  These capsules do not meet the FDA's standards for safety and effectiveness.

"There's no pill or capsule that can replace your sunscreen," FDA Commissioner Scott Gottlieb, M.D., said in a statement.

The agency specifically issued warnings about four products:  GliSODin's Advanced Skin Brightening Formula, Napa Valley Bioscience's Sunsafe Rx, Pharmacy Direct's Solaricare, and Sunergized's Sunergetic.

According to Gottlieb, these companies "are putting people's health at risk by giving consumers a false sense of security that a dietary supplement could prevent sunburn, reduce early skin aging caused by the sun, or protect from the risks of skin cancer."

The companies have been instructed to correct all violations and to review product websites and labeling to ensure that the claims they are making don't violate federal law.

CBS News reached out to the companies for comment but did not receive an immediate response.

© 2018 CBS Interactive Inc.  All Rights Reserved.