Federal drug regulators warned Procter & Gamble on Wednesday for adding Vitamin C to its Vicks cold formulas, a combination not allowed by federal regulations.
The Food and Drug Administration said medications like Vicks Nyquil and Dayquil are not permitted to contain the vitamin.
According to the agency, a panel of experts found "no study which demonstrated that vitamin C is unequivocally effective for the prevention or treatment of the common cold."
P&G advertising for Vicks says the vitamin "can help blunt" the effects of a cold.
In general, the FDA discourages firms from packaging drugs with dietary supplements because it gives the impression both have been evaluated by the FDA, when in fact, the agency only regulates drugs.
Vicks formula contains three drug ingredients: acetaminophen, a fever reducer; dextromethorphan, a cough suppresant, and phenylephrine, a decongestant.
Calls placed to the Cincinnati-based consumer-products maker were not immediately returned Wednesday.
The FDA's letter, posted online Wednesday, gives the company 15 business days to respond with a plan for addressing the violations.
The FDA regularly issues warning letters to companies that do not follow regulations for manufacturing and marketing. The letters are not legally binding, but the agency can take companies to court if they are ignored.
In recent years the FDA has begun cracking down on manufacturers who overstate the benefits of their products, amid increased demand for healthy foods.
