There is no scientific evidence that the diet removes any toxins, or that it achieves anything beyond temporary weight loss. Though unlikely to be harmful over the short term it can be harmful over the long term. Short term side effects include fatigue, nausea, dizziness, and dehydration, while long term harm includes loss of muscle mass. [...] Nutritionist Jane Clark points to a lack of essential nutrients in this program, citing a deficiency of protein, vitamins, and minerals. As a result of these deficiencies, including far fewer calories than the recommended amount for health and optimum functioning, individuals on the diet may experience headaches and a variety of other symptoms in the short term and the diet is potentially harmful over the long term. The program has been described as an extreme fad or crash diet, and any weight lost during the fast can be expected to be regained once the diet is stopped. Dietician Keri Glassman has said those following the diet are "guaranteed" to gain weight after stopping.
On a related note, two of my aunts went on a "intestine cleanse" (dunno the proper English term...?) as part of a yoga course we attended. I thought it sounded a bit too far out, so I opted out. The cleanse consisted of drinking insane amounts of salt water in a short amount of time while doing light yoga to accelerate the process which would inevitably end in a run for the bathroom. Afterwards they had to eat only steamed veggies for days. My aunts reported back the next week they had never felt as horrible and low in energy.