Don't depend on your doctor to always tell you everything you need to know about prescription drugs. Doctors are too busy these days; and have a hard enough time trying to keep up with advances in medicine.
If your doctor prescribed MiraLax for you, then go to the following website:
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X> MiraLax...About MiraLax -- FAQs -- Resources
http://www.miralax.com/miralaxlaxative/index.htm