On July 27, 2005, WLF wrote to DDMAC, calling on DDMAC to withdraw a warning letter sent to Pfizer, Inc. on July 20, 2005 regarding Pfizer’s allegedly improper promotion of Zyvox — based on an professional journal advertisement run by Pfizer. Zyvox is a drug used to treat skin infections and two types of pneumonia. DDMAC alleged that the advertisement made unsubstantiated effectiveness claims about the drug, including claims that Zyvox is superior to an alternative product. WLF’s letter to DDMAC alleged that DDMAC’s actions were unwarranted because Pfizer’s effectiveness claims were based on a clinical study whose findings DDMAC has no basis for challenging. WLF also faulted DDMAC for criticizing Pfizer’s comparative claims, because DDMAC based its criticism on a supposed rule that has never been properly promulgated by FDA. FDA may establish such rules only through notice-and-comment rulemaking, WLF argued.