Every few weeks, Dr. Tony Wing Lai Mak, a pathologist at Princess Margaret Hospital in Hong Kong, receives blood and urine samples from yet another patient hospitalized after taking a traditional Chinese medical or health supplement.

His toxicology lab finds the same culprits over and over: adulterants hidden in the dose.

Although the Hong Kong Department of Health regularly issues warnings about the medicines, “we’re still seeing this all the time,” Dr. Mak said.

“These are illegal products that are damaging to people’s health and can even kill. Yet somehow, they’re still here.”

The frequency and serious nature of the cases inspired Dr. Mak and his colleagues to compile a decade’s worth of observations, which they recently published in the British Journal of Clinical Pharmacology.

From 2005 to 2015, they reported, 404 people ranging in age from 1 month to 90 were treated at the hospital after taking tainted supplements.

Their sample, they note, comprises only those patients who took proprietary products processed into pills or other finished doses — not the considerable number who consumed whole, unprocessed plant or animal materials.

After testing 487 products handed over by sick patients, Dr. Mak and his colleagues discovered 1,234 hidden ingredients, including both approved and banned Western drugs, drug analogues and animal thyroid tissue.

Sibutramine, an appetite suppressant taken off the market after it was linked to cardiovascular problems, was the most commonly identified adulterant, found in 155 products.

Health supplements containing undeclared ingredients are illegal in Hong Kong, Dr. Mak said, but residents may purchase them in unscrupulous local shops, on the internet, or while visiting mainland China or abroad.

It is impossible to say how prevalent these contaminated supplements are in Hong Kong, Dr. Mak said. He only tests samples from patients whose adverse reactions are severe enough to require medical attention.

Many more may not seek help, or may not experience any side effects at all. “In China, we have so many products around and we take them all the time,” Dr. Mak said. “I do not know the denominator here.”

Chinese medicinal remedies are gaining popularity worldwide thanks to a perception that they are “natural and safe,” Dr. Mak said. A study of 2,600 proprietary Chinese medicine products in Taiwan found that a quarter were contaminated with synthetic drugs.

In the United States, the Food and Drug Administration has identified more than 800 adulterated dietary supplements on the market.