Washington ? 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.