SILVER SPRING, Md. (AP) — The U.S. Food and Drug Administration sent a warning letter Friday to Procter & Gamble Co. saying the company was illegally marketing two Vicks cold and flu medicines containing vitamin C.

The FDA said the single-dose combinations of drug and dietary ingredients in Vicks DayQuil Plus Vitamin C and Vicks Nyquil Plus Vitamin C cannot be marketed legally because they have not been proven safe and effective.

The agency also said it had previously determined that there was insufficient data to show vitamin C is safe and effective in preventing or treating the common cold.

Earlier this month the agency issued a similar warning letter but then quickly retracted it. FDA spokesman Christopher Kelly said Friday the letter was "issued prematurely — it had not completed our internal clearance process."

The FDA said it allows some over-the-counter drugs to be marketed without agency approval, but that the two Vicks products did not comply with its rules and first must be evaluated and approved under the FDA's new drug approval process.

A spokeswoman for the Cincinnati consumer products company noted in an e-mail message that the FDA warning was "not new news," referring to the earlier warning letter.

"It's also not about the safety or quality of the Vicks NyQuil or DayQuil brands," wrote Kate DiCarlo. "We believe we are marketing within FDA guidelines and regulations, and we'll continue to work closely with the FDA to resolve any concerns."