Miralax is a powder composed of polyethylene glycol. It is taken by dissolving a heaping tablespoonful in 8 ounces of water. It is not absorbed from the intestine. Rather, it draws water into the bowel which helps flush it out, thus relieving constipation.
Because it is not absorbed, Miralax is not toxic to any tissue or organ in the body. Another safety feature is that it does not appear to interact with other drugs, though it is theoretically possible that an overly brisk reaction to Miralax might speed a medication through the intestine before it could be absorbed.
Allergic reactions are uncommon. I could find only a single report of two cases of it causing a rash.
That Miralax can be obtained without a prescription is evidence that it is a fairly safe. Although most recommendations state that it should not be taken for longer than two weeks without a doctor's consent, I am unaware of any reports showing dangers of prolonged use.
I advise patients to drink plenty of water and to continue eating ample roughage while on Miralax. You should avoid excessive use. Intermittent dosing, perhaps three days out of the week, is unlikely to have any long-term unwanted effects.