Some are poisonous. Some can have toxic side effects if not properly balanced with herbs that counteract those effects. They should not ever be used indiscriminately without the proper guidance of a health care professional. They should not ever be used on a daily basis without the proper guidance of a health care professional. Many people feel that just because herbs are natural, they can't be hurt by them.
Nothing can be further from the truth.
Many people feel that if a little makes me feel better, a lot will make me feel fantastic. This is not the case with herbs. Dosages need to be carefully controlled, as with any medicine. You wouldn't swallow a whole bottle of antibiotics at one time to kill a bacterial infection. Apply the same caution to herbal medicines.