Taking laxatives for weight loss has a long history, but that doesn't mean it is good practice. In general, laxatives are designed to be taken for a limited period of time, for those with certain conditions or illnesses. These include:

When a person has a condition like a hernia or piles
Where there are chronic factors interfering with normal bowel movements (damage to the nerves in the spinal cord, hypothyroidism)
For those who are taking other medications that cause constipation (some painkillers and antidepressants)

Other than this, laxatives should only be used short term to relieve constipation. Long term use runs the risk of dependency - over time the body adjusts, so that eventually normal bowel activity will not be possible without the medication.

The other risk is that excessive use can cause diarrhoea or disrupt sodium levels in the body.

The usefulness for weight loss is highly dubious - taking a medication to evacuate the bowel is the equivalent of binging and purging. It is far better to eat less in the first place and let the body absorb all the nutrients it needs as the food passes naturally through your gut.

If you are constipated, you need to get more fibre in your diet. Good sources include:

wheat bran
unpeeled fruit
leafy green vegetables
wholemeal bread

Coincidentally, these are all excellent foods for those looking to lose weight! So ditch the drugs and start eating right.