Launched four years ago by a former San Diego cop with no formal medical training, Metabolife is now one of America's best-selling herbal products. Enthusiasts peddle it from living rooms and kiosks, as well as retail shops. And Metabolife International, the privately held company that produces the stuff, says its sales will approach $1 billion this year--a figure that places Metabolife in the same league as such blockbuster pharmaceuticals as Prozac and Viagra. The catch is that the stimulants it contains can be dangerous: the Food and Drug Administration has warned that products like Metabolife can cause "cardiac arrhythmia and death." Several states have moved to ban or restrict their use, and a few users have filed lawsuits claiming injury. Yet consumers continue to down an estimated 225,000 pills every hour.