The Food and Drug Administration's(FDA) new effort to re-examine very old drugs that began selling before the government ever required proof that drugs worked well, has landed dozens of brands of prescription cough syrup in trouble. Until now FDA stressed that the expectorant, extended-release guaifenesin, is not dangerous. However, the recent crackdown lead to the FDA formally approving sale of Adams Laboratories' Mucinex, making competing companies' unapproved versions automatically illegal. The FDA has warned 66 manufacturers that Mucinex would be the only once-a-day prescription guaifenesin allowed to sell unless its competitors won formal approval. The FDA has given these companies time till end of November to pull their brands of extended-release guaifenesin off the market.