“This is a fairly typical instance of the public security authorities intervening in a civil dispute; it’s an overreach of power,” Wang Yong, a professor of commercial law at the China University of Political Science and Law in Beijing, said in an interview.

The number of cases of Chinese companies mobilizing the police against critics or commercial rivals has shrunk in recent years, he said.

“But in this Hongmao Medical Tonic case, the role of the public security authorities in local protectionism has again come to the fore,” Professor Wang said. “The fundamental problem is that for a long time in Chinese law, the boundary between civil and criminal cases has been blurred.”

Dr. Tan never expected such a ruckus after he published his essay criticizing Hongmao in December. He initially shared his views on a blog, which he said only a few fellow doctors read. He then spread it on WeChat, a popular Chinese social media service, where it was viewed more than 2,000 times, according to the police.

But the title of Dr. Tan’s article was eye-catching: “China’s miracle liquor, ‘Hongmao Medical Tonic,’ a poison from heaven.” Dr. Tan argued that the supposed curative effects of the 67 ingredients said to be used in the alcohol-based elixir were unclear at best, and could be dangerous for people with high blood pressure or diabetes.

Hongmao had brushed aside these medical concerns, and its plethora of ads on Chinese television have exaggerated its powers, Dr. Tan wrote. Citing traditional Chinese medical theories, it claimed to relieve ailments like painful joints, frail kidneys, and weakness and anemia in women by combining 67 ingredients from plants and animals.

His essay had little initial impact. But in January, when Dr. Tan entered an elevator to his apartment, two men jumped in and showed police badges. They revealed that they were officers, sent from Inner Mongolia to detain and question him. Later, they took Dr. Tan to Liangcheng County by train and a long car ride.