Laxative abuse is one of the 'compensatory behaviours' associated with Bulimia Nervosa. Sufferer's can consume excessive amounts of laxatives (numerous packets at a time) in an attempt to purge themselves of the food consumed after a binge, and to get rid of 'unwanted calories'. 

Abuse of laxatives is incredibly dangerous, and can have serious health implications but unfortunately, due to the secretive and sometimes perceived embarassing nature of the compensatory behaviour, many sufferer's will not seek help, and can get caught in quite an addictive cycle of use. 
Unfortunately, abusing laxatives creates the mistaken belief that the individual is actually losing weight, especially as straight afterwards the laxatives have 'taken effect', the sufferer can in fact weigh less. However, this is only water weight, and the majority of the calories have already been absorbed by the time the laxatives can take effect. This artificial weight loss will just be re-gained as soon as the sufferer hydrates themself again. 

The Dangers: 

Dehydration: Laxatives can cause severe hydration, as they essentially drain all the water in your system to take effect. This can have severe health implications, including kidney infections or damge, fainting and in some cases death.
Electrolyte & Minerals: Laxative abuse can upset your electrolyte and mineral levels, including sodium, magnesium and potassium which can cause damage, and impair the functioning of vital organs.

"I spent years and years abusing laxatives, under the mis-illusion that they were in fact helping me to lose weight, but as time went on, they became more of an addiction or obsession than anything else. I couldn't imagine the thought of not taking them in the evening, and I would even take them when I had not eaten anything that day. I was completely out of control. Even when I knew that they only got rid of water, and the calories had already been absorbed, this did not stop me. 
It took a lot of dedication and help to break the laxative abuse cycle"