Herbal remedies are not a benign alternative to mainstream medicine, two professors of medicine claim in a recent study, and may represent a "global hazard" to people's health.
Herbal treatments are taken around the world and are often prescribed by traditional healing practices. Some have been in use for hundreds, or even thousands of years.
But "the history of herbal use also shows that not all herbs are benign and sometimes are deadly," wrote the paper's co-authors, Arthur Grollman and Donald Marcus.
"Moreover, we cannot know whether all herbal medicines are safe, because only a few have been tested systematically for toxicity or carcinogenicity."
Grollman is a professor of pharmacological sciences at Stony Brook University, and Marcus is a professor emeritus at Baylor College of Medicine in Texas. Both are physicians.
They published their commentary this month in the peer-reviewed journal EMBO Reports.
Marcus told CNBC that the authors are particularly concerned about the widespread use of herbs in Asia and Africa, and the encouragement of their use by governments, public health officials and the World Health Organisation.
"WHO is not only ignoring the hazards of herbals - it is promoting them," he said, adding that governments in Africa and Asia are expanding the production, marketing and export of botanical products.
Grollman and Marcus focused their attention on the use of herbal medicines globally, but their comments evoked the ire of some in the US dietary supplement industry who characterize the doctors' critical language as over-the-top.
Duffy Mackay, senior vice president of scientific and regulatory affairs at the Council for Responsible Nutrition, a Washington lobbying group for the industry, said that while his group favors more research into botanicals, calling herbal remedies a "global health hazard" is "fear-mongering."
"Our organisation and our members are all in favour of more research into the safety and efficacy of plants," he told CNBC. "And we support a variety of initiatives, and we think botanicals play an active role in people's everyday health and maintenance."
Mackay read the paper and said he thinks Marcus and Grollman overly favour pharmaceuticals, which seems to be "at the expense of having any open mindedness that any of these other medical structures, such as Chinese medicine" or traditional Indian medicine "would have any benefit, or that any of these plants would have any benefit."
Herbal use in the United States and Europe is also common.
"One of the things about it that motivates us is that the promotion of herbal remedies, not just in Asia and Africa but also the United States, is (the contention) that 'If they have been around for hundreds of years, they must work and they must be safe.'" Marcus told CNBC.
"Well it is clear that they aren't safe, and there is relatively little evidence that most of them work."
The authors also note that several herbs commonly available in the US were shown to be ineffective in recent trials.
These include "gingko biloba, used to prevent decline in memory and onset of dementia, St John's wort for treating minor or moderate depression, garlic to reduce hypercholesterolemia, saw palmetto for symptoms of prostatic enlargement, and echinacea to prevent or treat the common cold."
Mackay of CRN disputed that, however, saying that there have in fact been studies that indicate efficacy for some herbs.
He said the doctors' claims don't "do true justice to the literature.
There are many trials that show that St. Johns' wort is as effective as antidepressants for mild to moderate depression. There are many trials that Gingko has an impact on cerebral blood flow which we know is an important end point."
A very big industry
Herbals are a huge business, part of the $34 billion supplements industry in the United States. Marcus told CNBC he figures "about half" of that industry is herbals and related products, as opposed to vitamins or other supplements.
He told CNBC his criticism is not focused on vitamin supplements, and more on herbal remedies that have not been evaluated for safety, especially as they are used outside the US
But some critics say that even in the United States, herbals are insufficiently studied and regulated.
"The biggest problem is that this is an unregulated industry, which is to say there is no way of knowing if what is on the label is actually in the bottle," said Paul Offit, a doctor at Children's Hospital in Philadelphia and a critic of the supplement industry, who was not involved with the study.
If regulators held supplements, including herbals, to the same safety and efficacy standards as pharmaceuticals, Offit said many products would vanish from the marketplace.
"If you regulated this industry, it would disappear."
"The main thing to do is to get together traditional healers, pharmacologists and biochemists, and public health authorities," Marcus said, "and try to get people to accept the idea that herbals are not innocuous and we need to start gathering data on which herbals are causing disease."