It began with menacing phone calls, strange malfunctions of the office computers, and men in parked cars photographing the entrance to the small consumer advocacy group's offices. Then at dusk one day last December, Dr. Esperanza Cerón, the head of the organization, said she noticed two strange men on motorcycles trailing her Chevy sedan as she headed home from work. She tried to lose them in Bogotá's rush-hour traffic, but they edged up to her car and pounded on the windows. "If you don't keep your mouth shut," one man shouted, she recalled in a recent interview, "you know what the consequences will be."

The episode, which Dr. Cerón reported to federal investigators, was reminiscent of the intimidation often used against those who challenged the drug cartels that once dominated Colombia. But the narcotics trade was not the target of Dr. Cerón and her colleagues. Their work had upset a different multibillion-dollar industry: the makers of soda and other sugar-sweetened beverages.

Their organization, Educar Consumidores, was the most visible proponent of a proposed 20 percent tax on sugary drinks that was heading for a vote that month in Colombia's Legislature. The group had raised money, rallied allies to the cause and produced a provocative television ad that warned consumers how sugar-laden beverages can lead to obesity and diet-related illnesses like diabetes. The backlash was fierce. A Colombian government agency, responding to a complaint by the nation's leading soda company that called the ad misleading, ordered it off the air. Then the agency went further: It prohibited Dr. Cerón and her colleagues from publicly discussing the health risks of sugar, under penalty of a $250,000 fine.