Such allegations concerning Merck's actions regarding Vioxx stand in stark contrast to Merck's reputation that resulted from its actions with another of its drugs. Mectizan is a Merck drug that prevents river blindness, a disease preva¬lent in tropical nations. Caused by parasitic worms spread by insects living along tropical rivers, river blindness infects millions of people annually, causing severe rashes, itching, and loss of sight. A single tablet of Mectizan administered once a year can relieve the symptoms and prevent the disease from progressing.