Herbal medicine is used to treat many conditions, such as allergies, asthma, eczema, premenstrual syndrome, rheumatoid arthritis, fibromyalgia, migraine, menopausal symptoms, chronic fatigue, irritable bowel syndrome, and cancer, among others.t is best to take herbal supplements under the guidance of a trained health care provider. For example, one study found that 90% of arthritic patients use alternative therapies, such as herbal medicine.