The country's largest organization of doctors Tuesday refused to challenge a controversial tobacco bill that would ban many flavor additives in cigarettes sold in the United States but exempt menthol flavoring. Under a bill before Congress that gives control of tobacco products to the Food and Drug Administration, flavor additives such as mint, clove and vanilla, which appeal to young people, would be banned, but menthol flavoring would be allowed. Menthol is preferred by more than 75% of black smokers, according to government estimates. Fewer than 25% of whites smoke it.