Complementary medicines

Last updated: February 07, 2020

The amount of reliable safety information available about herbal medicines can be very limited. This lack of data makes it difficult to provide information in response to common clinical questions such as: Will it interact with my prescription medicines? Is it OK to use in renal impairment? Could it be causing my patient's hyperglycaemia? The online information sources we highlighted at the beginning of this section can help you by summarising published evidence or offering guidance.
Questions about interactions between conventional medicines and herbal products are particularly common. Since there are rarely any high quality clinical studies, it is helpful to look for herbal side effects that might be additive to those of the medicine or, conversely, may oppose its therapeutic action. For example, high strength garlic products can increase the risk of bleeding so patients should be careful about taking anticoagulants or antiplatelet medicines. Echinacea has been reported to have immune stimulating effects so caution is advised if taken with immunosuppressant medicines. A few herbal medicines such as St John's wort and Panax ginseng may affect drug-metabolising enzymes such as cytochrome P450, which broadens the potential for interactions.

Many side effects of herbal products are probably dose-related Type A reactions, but the constituents of the herb and/or the pharmacology of those constituents are often not clearly established. So predicting side effects is not easy, and large scale safety studies are generally absent. Consequently, a lot of the information about safety comes from small studies or case reports of suspected significant adverse reactions to individual herbal medicines in the medical literature. People can of course be allergic to herbal medicines too – Calendula, for example, can cause eczema-type skin reactions. The main method for reporting adverse reactions due to herbal medicines in the UK is to the MHRA through the Yellow Card reporting scheme.

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