"Are there any herbs you should avoid making tea from?"
Some of those I'm describing in this post. Thank you for your question, Owen! :)
Photo of Comfrey herb - courtesy of ESOX LUCIUS
First of all a couple of general rules to follow - just to be on the safe side:
- only use an herb if you know for sure what it is - some plants are poisonous
- when buying, choose a reliable merchant that sells organic, fair-trade herbs
- avoid using tea that causes unpleasant reactions - unless you're under supervision of a licensed practitioner
- pregnant and lactating women should completely stay away from certain herbs
Photo of Lobelia nicotianifolia courtesy of dinesh_valke
Although Comfrey herb (top photo) historically has many uses for topical applications, it also contains certain alkaloids that could cause permanent damage to liver when used for an extended period of time/continuously.
Lobelia herb contains toxins that are similar in effect to those of niccotine.
These warnings and sometimes even the lists of prohibited plants vary from country to country. According to Wiki "The UK currently does not require natural products such as herbs to have any evidence concerning their efficacy, but does treat them technically as food stuff and require that they are safe for consumption."
Photo of Poison Ivy - courtesy of Martin LaBar (going on hiatus)
You might have heard about the Poison Ivy plant becoming more and more poisonous and causing stronger allergic reactions than ever before. Some researchers suggests that it might be thriving on polluted air. As an interesting fact, in 1900' this same plant was commonly sold as a decorative house plant for its beautiful leaves...
Similar, from reading different reports on the same herb from different parts of the world, I realized that a plant that has a pretty minor warning like "it might cause skin irritation to people with sensitive skin" sometimes has a potential to become a much stronger irritant when growing in subtropical climate conditions. An example is Rue herb (Ruta graveolens) - another beautiful plant.
Photo of Rue herb - courtesy of Luigi FDV
Another beauty out here is Foxglove, often grown as an ornamental plant for its showy flowers. However, the entire plant is poisonous (a nibble is said to be enough to potentially cause death), including seeds and roots. At the same time, a group of active compounds are extracted from (mostly) the leaves of this herb to produce a medicine by the name of "Digitalin".
Photo of Digitalis purpurea (Common Foxglove, Purple Foxglove or Lady's Glove) - courtesy of Mclaire2
The last plant I'd like to highlight in this post is Poison Oak. Named most likely for the oak-like shape of its leaves and very unpopular in parts of northern California/west coast of U.S. - because of its toxicity that can cause a severe skin rash. It can grow as a shrub or a vine.
Photo of Poison Oak - courtesy of goingslo
In short, different plants have different effects on different people - therefore using herbs for medicinal purposes should be supervised by practitioner who is trained and licensed.