why can dulcolax not be crushed or chewed?

Dulcolax pills have a special 'enteric coat' that is designed to prevent the pills dissolving in the acidic environment of the stomach. The pills pass through the stomach intact and only dissolve when they reach the large intestine where the active drug is needed.
If the Dulcolax tablets are crushed, chewed or broken, the coating will be damaged and the pills won't work as well.