Excerpt from Rosemary Gladstar's Medicinal Herbs
"I once heard a doctor state that the "side effects" of pharmaceutical medication are not side effects at all, but the actual effects of the medication. This is an aspect I appreciate about herbal remedies; they are effective, yes, but side effects are few and far between. There are people who have idiosyncratic reactions to certain foods and herbs, but it's an individual reaction rather than toxicity in the plants. And while there certainly are toxic plants that can have nasty "side effects" or actions, most of these herbs are not legal for use and certainly are not used in herbal family medicine. In this book you'll find none of the herbs with a potential for toxicity, but rather those congenial herbs with a long history of use as food and medicine with few or no known side effects.
When a person does experience "side effects" from a particular herb, they are generally short term and idiosyncratic: itchy eyes, sore throat, a skin rash, or nausea and an upset stomach. These symptoms disappear after discontinuance of the offending herb and are not long lasting.
Because we are working with nontoxic herbs with few or no side effects, we don't have to be as careful with exact dosages. The problem is generally not taking enough of the herbs to be effective, rather than taking too much."