By swelling up within your intestines, bulk-forming laxatives soften and increase the volume of your stools. This then encourages your bowels to move and push the stools out. The full effect of this type of laxative can take a few days to occur.
Stimulant laxatives work by speeding up the movements of the intestines. They take 8-12 hours to work. Osmotic laxatives work by reducing the amount of water absorbed from the bowel and so increasing the amount of water in your stools. This makes them softer and easier to pass.
Using laxatives to lose weight is dangerous - in the short term and the long term. The food isn't digested so the body is not absorbing the vitamins and minerals you need.
You can become anaemic, dehydrated and ill quite quickly.
Over the long term, your bowel muscles don't work properly, and can cease to function correctly, resulting in uncontrollable feceal leakage.
Laxatives are excellent short term treatment for the relief of constipation. That is the only safe way to use them.