As herbal teas can be composed of any plant material, including some plants that are known to be toxic, the specific ingredients must be checked individually for health and safety. Most retail herbal teas sold as beverages could be considered safe, but medicinal herbal teas could easily contain herbs that cause damage in large amounts.
While most herbal teas are safe for regular consumption, some herbs have toxic or allergenic effects. Among the greatest causes of concern are:
Comfrey, which contains alkaloids that can cause permanent liver damage with chronic use
Lobelia, which contains toxins similar in effect to nicotine
Herbal teas can also have different effects from person to person, and this is further compounded by the problem of potential misidentification. The deadly foxglove, for example, can be mistaken for the much more benign (but still relatively toxic to the liver) comfrey.
The UK does not require herbal teas to have any evidence concerning their efficacy, but does treat them technically as food products and require that they are safe for consumption.
Mint and peppermint herbal teas had significantly stronger ferrous ion chelating ability than true teas