Herb practitioners in different parts of the world, including India, mix different combinations of plant extracts to treat ailments, including asthma and arthritis.
The researchers looked at 1,300 published articles on the subject and analysed the only three found to be randomised clinical trials.
These three trials investigated the treatment of osteoarthritis of the knee (a knee joint disease), irritable bowel syndrome (a most common gastrointestinal disorder) and the relief of side effects caused by drug treatment for cancer, reported the online edition of Daily Mail.
The researchers from Peninsula Medical School, Exeter, found that in two of the studies, there was no statistical difference in the results from tailored herbal medicine and a placebo, or dummy treatment.
Tailored herbal medicine therapy seemed to work better than a placebo for irritable bowel syndrome, but was not as effective as giving patients standard herbal preparations.
"Our conclusion is, therefore, there is no convincing evidence that individualised herbal medicine works in any indication." Researcher Peter Canter said in his article in the Postgraduate Medical Journal.
In fact, he said, there were serious safety risks involved, such as adulteration of ingredients with poisonous or banned substances, and interaction with other herbs or medicines.
Canter said many studies did promote the effectiveness of herbal medicines, but most were examining standard preparations or single herb extracts.