The active ingredient in MiraLAX, polyethylene glycol 3350, can take 2 to 4 days to produce a bowel movement, according to MedlinePlus. Polyethylene glycol 3350 is an osmotic laxative that works by drawing water into the stool, softening it and making it easier to pass.
MiraLAX and other similar formulations are produced in a powdered format that is mixed in with liquid and then consumed orally, according to MedlinePlus. The usual course of treatment is to take the recommended dosage once daily as suggested by a doctor or recommended on MiraLAX's label for a period of 2 weeks. It is only recommended for the treatment of occasional constipation.
MiraLAX bills itself as a way to "unblock your system naturally" without any bloating, gas, cramping or sudden urgency that might be experienced with other types of laxatives. MiraLAX's website also states that it works to hydrate, ease and soften the stool.
Drugs.com warns that users should check with their doctors prior to use if they are pregnant, breastfeeding or planning on becoming pregnant or if they are taking any other medications. Although Drugs.com states that there are no known interaction with other medicines and polyethylene glycol 3350 as of 2014, some medicines may interact with it.