Rare Reactions in Sensitive Individuals
A few herbs are photosensitive—if you go out in the sun after ingesting them, you might have a skin reaction. There are very few such documented cases, but reactions generally occur within 24 hours and only in individuals who are especially sensitive. Even some very common fruits and vegetables, such as figs and carrots, can cause this reaction. Don quai, the related angelica and yarrow are potential herbal culprits. Lovage causes photosensitivity in animals but apparently not in people (although anyone with weak kidneys should not use it). Applying bergamot essential oil to your skin can also cause this reaction, unless you get a version of the oil from which the reactive compound, bergaptene, has been removed (it will say "bergaptene-free" on the label). Other citrus essential oils, especially lemon, are said to sometimes cause a photo- sensitive reaction.A number of fresh plants, such as goldenseal, blue cohosh, mullein, elecampane, motherwort, parsley, hops and dandelion, can cause itching or a skin rash in sensitive individuals. This reaction is rarely more than bothersome, and if you react to one of these plants, this does not necessarily mean that you will react to the others.
Some herbs—cayenne, for instance—can cause a burning sensation on sensitive skin. Researchers at the University of Chicago Hospital dubbed cayenne's painful sting "Hunan hand" because cooks working in Chinese restaurants often experience it. These researchers found that soaking the peppers in vinegar for a few hours neutralizes the reaction. Fat and alcohol can be used to relieve the pain, so next time you accidentally chomp down on a hot pepper camouflaged in a plate of Hunan bean curd, try taking a sip of milk or beer. Garlic and other pungent herbs can irritate and even burn sensitive skin. Large amounts even make some people's eyes more sensitive to light.
It is probably obvious, but if you are a hemophiliac or are scheduled for surgery, you should avoid herbs that have anticlotting properties. These include turmeric, alfalfa and motherwort. Ginseng, feverfew and garlic also thin the blood to some degree.
The immune herb baptisia contains a substance that is known to be toxic to animals in large amounts—it has poisoned animals who have overgrazed on it—but I could find no reports of it producing a toxic reaction in people. Baptisia is commonly used in Sweden, but is classified as a drug there.