http://www.something-fishy.org/dangers/methods.php

Laxatives
Pills or liquid, herbal or otherwise, laxative abuse is a common form of "weight control" in people suffering with Eating Disorders. The use of laxatives as a way to lose or control weight is not only dangerous, but irrational. The actual purpose of taking a laxative is to stimulate the large bowel to empty, which occurs only after the food and it's calories for energy have been absorbed through the small intestines. Essentially, a person taking laxatives to control weight is only going to cause their body to lose fluid, which can potentionally be followed by periods of water retention and an appearance of bloating (causing the sufferer to want to lose more weight and ingest more laxatives). The reason people suffering from eating disorders seem to lose weight while taking laxatives is because in most cases they are increasingly watching calorie intake and restricting food consumption, inducing vomiting, and/or compulsively exersizing.
Continued laxative use can cause bloating, cramping, dehydration, electrolyte disturbances and imbalances, cardiac arrhythmias, irregular heart beat and heart attack, renal problems, and death.
Phenolphthalein, a popular ingredient previously found in many brands of laxatives has now been recalled by the FDA (Food and Drug Administration, United States) due to studies indicating that it has been linked to cancer.
Withdrawal symptoms can be expected in stopping the use of laxatives after a continued period of using them as a "weight loss" method. These include, abdominal cramping, mild to severe constipation, bloating, mood swings and general feelings of fatigue and "feeling sick." In less severe cases the symptoms will usually subside in about 2 weeks, but in cases where a person have ingested handfuls or more laxatives on a regular basis, it may take longer and require medical assitance.