Bulimia nervosa is an eating disorder. People who have it eat very large amounts of food (binging) and many also use inappropriate means to rid their bodies of the food (purging). Purging may be caused by vomiting, laxatives, or water pills. Individuals use this cycle to prevent gaining weight. These behaviors last for more than three months for at least two times a week. Individuals are overly concerned with weight and body image. In some, excessive exercise or fasting may replace or supplement purging.