FILE PHOTO: The headquarters of the U.S. Food and Drug Administration (FDA) is shown in Silver Spring, Maryland, November 4, 2009. REUTERS/Jason Reed/File Photo

(Reuters) - The U.S. Food and Drug Administration has told four companies to remove 44 of their flavored e-liquid and hookah tobacco products that do not have the required approval for sale in the U.S., the agency said on Thursday.

The move comes against the backdrop of the FDA’s efforts to curb the usage of the addictive substances among young adults.

The agency said it has issued warning letters to Mighty Vapors LLC, Liquid Labs USA LLC, V8P Juice International LLC and Hookah Imports Inc and has sought their response within 15 days.(reut.rs/2YUeBn7)

The FDA said the products had been introduced into the market after the effective date of a rule that extended FDA’s authority to all tobacco products, thereby making them adulterated or misbranded.

E-cigarettes have existed in a regulatory gray area for years and a U.S. federal judge in July ordered the FDA to implement a 10-month deadline for submitting a formal application by e-cigarette makers to keep their products on the market.

E-cigarettes are generally thought to be safer than traditional cigarettes, but the long-term health effects of the nicotine devices remain largely unknown.

The health agency previously asked nearly 90 makers of Electronic Nicotine Delivery System products for information, including evidence that the product is legally marketed and a number of companies have removed their products from the market.