Already wounded by the withdrawal of its Vioxx pain reliever from the market, Merck & Co. must now contend with hundreds of lawsuits over the drug’s side effects -- lawsuits that threaten to further damage the company’s finances and reputation. Wall Street analysts are concerned about Merck’s potential legal liability. This week, Standard & Poor’s Corp. warned that it might downgrade its ratings on Merck’s debt because of the huge payouts the company might be forced to make.