If you pop a vitamin C tablet in your mouth, it's a good bet it came from China. Indeed, many of the world's vitamins are now made in China. In less than a decade, China has captured 90 percent of the U.S. market for vitamin C, driving almost everyone else out of business. Chinese pharmaceutical companies also have taken over much of the world market in the production of antibiotics, analgesics, enzymes and primary amino acids. According to an industry group, China makes 70 percent of the world's penicillin, 50 percent of its aspirin and 35 percent of its [Tylenol], as well as the bulk of vitamins A, B12, C and E. In the wake of a pet-food scandal, in which adulterated wheat gluten from China led to the deaths of thousands of pets in North America, and other instances of food and toothpaste tampering, China's vitamin producers are reaching out to reassure U.S. consumers that their vitamins are safe. Whether that's true isn't clear, however. Foreign food-safety experts say China's larger companies have reputations to protect. The question is how they maintain quality control. Since U.S. laws don't require food and drug sellers to label products with the country of origin of ingredients, it's impossible for consumers to know where food or supplements are coming from, not to mention what factory produced them.