Is it possible to overdose on miralax (polyethylene glycol)? my doc told me to take it twice a day mixed with prune juice. is this safe even if it says once on bottle?

Yes,I don't think so: I personally would not take MiraLAX (polyethylene glycol) even once a day. While most people seem to tolerate it well, it has the potential of causing very serious side effects ;, like all medicines, the risk of side effects increases with higher doses. I advise far safer alternatives such as triphala (an herbal blend). See http://www.Gutsense.Org/gutsense/the-role-of-miralax-laxative-in-autism-dementia-alzheimer.Html.