Unpredictable adverse effects of herbal products

Herbal products are being increasingly used all over the world for preventive and therapeutic purposes because of the belief of their safety. They have become an important part of health care system in many countries since they can easily be purchased in the health food stores or online. However, the lack of sufficient study on their efficacy and toxicity, inadequate controls of their availability, reduce their safety. Unlike conventional drugs, herbal products are not regulated for purity and potency. Herbal products contain substances which can induce or inhibit enzymes that take part in drug metabolism. Therefore the concurrent use of drugs with some medicinal plants can cause serious adverse effects and can also decrease the efficacy of the therapy. Particularly, drugs with narrow therapeutic index and plants which can affect drug metabolizing enzymes when used together, may lead to unpredictable adverse reactions. Impurities, contaminants and adulterants found in the herbal products, are the most common malpractises in herbal raw-material trade. In this review the unpredictable adverse effects of herbal products due to their possible interactions with drugs and also due to the adulteration and contamination with prohibited chemicals will be discussed in detail.