The FDA is warning Procter and Gamble about putting vitamin C in products like Nyquil and Dayquil. The federal regulators say combining vitamin C with Vicks cold formulas has not been approved. The company has marketed the Vicks cold remedies by saying vitamin C does not cure the common cold, but "can help blunt" symptoms. The regulatory agency says it does not have enough scientific proof that vitamin C is safe and effective in preventing or treating the common cold.