Laxatives do not cause weight loss. Any difference you might see on the scale is simply water weight. Calorie absorption occurs in the small intestine and laxatives empty the waste left in the large intestines so it isn't affecting how many calories you a taking in.
Basically you are losing poo weight that will come back as soon as you stop taking them and rehydrate. It's useless, a waste of money, painful, and not pleasant to have buttpee leaking out periodically.