Eating a small amount of a plant may not always be a problem, but large or repeated doses may be extremely harmful. There is no easy test for knowing poisonous plants from those that are safe. Many plants cause nausea, vomiting, abdominal cramps and diarrhea. Some plants have substances that are very irritating to the tongue, mouth and skin. Immediate burning pain in the mouth along with swelling is common.
Some plants can cause a skin rash.
Sometimes the rash only occurs after being in the sun or gets worse with exposure to sunlight. Don't assume a plant is safe for humans just because animals or birds eat it without harm. Heating and cooking do not necessarily destroy the toxin in a mushroom or a plant. Scientific studies have not been done on most herbal products. Herbal products are not regulated by the Food and Drug Administration for purity, potency and effectiveness.