I remember reading an article about a woman who lost a lot of weight following a sensible low-fat diet, then the weight loss ground to a halt. It turned out that because she wasn't taking in enough fat her body had gone into starvation mode, even though she was taking in enough actual calories. Apparently the body needs fat to help keep the metabolism ticking over, so if you take the tablets with an already low fat diet or carefully balanced VLCD it could have the opposite effect by eliminating the essential fats that your body needs to function.