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A commonly used herbal remedy that is banned in the U.S. and many European countries may still be harming users in Asia.

Since researchers connected an epidemic of kidney disease among Belgium women in the early 1990s to herbal medicines from a weight loss clinic, scientists have monitored the remedy for other potential health risks.

During the initial outbreak, scientists traced the kidney problems to aristolochic acid (AA), which belongs to a group of plants known as birthwort or Dutchman’s pipe. The agent is used for weight loss and to treat asthma and arthritis. But since the epidemic, herbs with aristolochic acid have been banned for medicinal uses in many countries, including the U.S. Researchers at King’s College London, however, now report in the Annals of Internal Medicine that millions of people are still being exposed, especially in Asia.

The scientists, led by Graham Lord, the director of the National Institute for Health Research Biomedical Research Centre at Guy’s and St Thomas’ NHS Foundation Trust and King’s College London, found that herbal medicines containing the acid are still found in China and other Asian countries and can be purchased online. “The reason we wrote this paper is to provide a diagnostic classification for aristolochic acid nephropathy (AAN) [the type of kidney failure associated with the agent]. For countries that haven’t asked the question of whether this is present, here is diagnostic criteria. We just don’t know what the levels of exposure are throughout the world,” he says.

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After reviewing 42 case studies and one trial related to caring for the disease, Lord and his colleagues came up with guidelines to help doctors to recognize cases of AAN, as well as treat the disease’s symptoms. They concluded that aristolochic acid could be linked to a variety of kidney diseases and urothelial cancer. More research, however, is needed to improve understanding of the risks of aristolochic acid. They write:

We see an urgent need for research addressing many key areas, including determining the true worldwide extent of exposure; deﬁning genetic variants that might confer increased sensitivity or resistance to the nephrotoxic effects of AA; testing the accuracy and utility of diagnostic criteria and optimum screening strategies, including the use of noninvasive biomarkers; and developing therapeutic agents that can reverse or delay progression of the disease. An international herbal reference center equipped to assess the composition and risks of products available to consumers would provide substantial public health beneﬁt.

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And given the wide availability of aristolochic acid, on the internet and in Asia, the authors say holes in regulation of herbal remedies need to be addressed. “The globalization of the distribution of these substances makes it very difficult to control,” says Lord.

Lord urges everyone using herbal medicine to be cautious. Active agents in familiar treatments can change over time, or new information on the risks and benefits of specific ingredients may also become available. Checking the ingredients is an important and often overlooked step. “Generally, a lot of people don’t ask. There are a lot of ingredients in [the remedies] that can change over time because they are complex. You need to check each time. It never hurts to check.” And, when in doubt, ask a physician about whether an herbal remedy is safe to take.