“Because it’s an all-natural herbal product, the F.D.A. does not want me to quote anyone saying anything in the product description that would insinuate that it treats, mitigates or cures any diseases,” she said.

GuruNanda is a California-based essential oils company named for its founder, Puneet Nanda, a self-described “entrepreneur-turned-yogi.” In an email, Megan Brown Bennett, a spokeswoman for GuruNanda, said that after the company became aware of the warning letter, it “immediately removed” any information related to treatment or prevention of Covid-19 and the coronavirus.

“GuruNanda, at all times, strives to be compliant with the law and will continue to work with the F.D.A. and the F.T.C. to ensure compliance with the applicable laws and regulations,” she said.

In an email, Jennifer Hickman, the owner of Vital Silver in Florida, said she was “unaware that my company was violating FDA standards, or that any of the statements could be considered fraudulent.” She added that she had removed all statements concerning Covid-19 from the company’s website and social media.

The other companies did not immediately respond to a request for comment.

The Jim Bakker Show broadcasts an hourlong show of the same name, hosted by the televangelists Jim and Lori Bakker. In 1989, Mr. Bakker was sentenced to 45 years in prison and fined $500,000 for fraud in connection with his TV ministry, and he ultimately served five years in prison and a halfway house.

Also on Monday, the Justice Department warned makers of health care products not to use the coronavirus outbreak to illegally profit from the sale of face masks, sterile gloves and other items.

“The Department of Justice stands ready to make sure that bad actors do not take advantage of emergency response efforts, health care providers or the American people during this crucial time,” Attorney General William P. Barr said in a statement.