Smokeless tobacco products remain far less popular than cigarettes in the U.S. Yet a collection of products that deliver nicotine without smoke—including dip, chew, snuff and newer items that look more like chewing gum—have sparked a heated debate about health risks.

Opponents of these products have presented numbers that suggest smokeless tobacco is an enormous public-health threat akin to cigarettes, while supporters, including some scientists, suggest smokeless items could offer a solution to smoking's toll on public health....