For its part, Merck has suggested that a relaunch may make sense due to the unique characteristics of Vioxx. For starters, the company has always pointed to Vioxx as the only Cox-2 inhibitor that has been labeled as safer on the stomach than older painkillers. In addition, it is the only Cox-2 drug that can be used by patients with certain allergies. Some patients have been pleading for a return of the drug despite its cardiac risks. Even so, Merck could face an uphill battle if it actually decides to pursue the drug's return. "Given that Merck decided voluntarily to withdraw Vioxx, the possibility of re-launching was always present," wrote Lehman Brothers analyst Anthony Butler, who has a neutral rating on the stock. But, "We believe that there are likely to be multiple hurdles for Merck to overcome should they decide to re-launch." Prior to its recall, Vioxx ranked as one of the most heavily marketed -- and prescribed -- drugs in the country. It once generated about $2.5 billion in annual sales.