There's an old wives tale that using castor oil would induce labor... I was warned of this by my doctor after my due date came and went.
He said all it does is make you poop... a lot. There's nothing magic about it; it's strictly a laxative.
As others have pointed out, laxatives are not a way to lose weight. First off, you're not even "purging" because your body has already absorbed nutrients. And personally, I think it's a pretty harsh laxative anyway.