To hear Merck executives tell it, the $4.8 billion settlement of Vioxx claims is practically airtight. Of course, that's what American Home Products executives were saying back in 1999, when they sewed up a $4.8 billion settlement of thousands of lawsuits over the dangerous diet drug combination fen-phen. If previous mass tort "settlements" are any example, Merck will be spending a lot more than $4.8 billion before it's done. American Home Products, now part of Wyeth, has reserved $21.1 billion so far to pay for diet drug litigation, and the settlement is in its seventh iteration. Merck has better facts on its side--it pulled Vioxx off the market early on, and it's unlikely many more plaintiffs will emerge before state statute of limitations laws doom their claims--but the company could easily wind up spending two to three times as much as it expects.