18 yr old son hasnt had a bm in 3 days i gave him miralax (polyethylene glycol) for 2 days and still nothing should i give him something else? he has left side discomfort.

Med takes longer: Miralax (polyethylene glycol) is a wetting agent not a laxative. Relief from the discomfort of constipation is best done with a washout of the rectal vault & sigmoid colon using an enema or stimulant suppository like ducolax. Miralax (polyethylene glycol) will take about 3 days to work its way through the system if the kid is having bowel movements.It's longer term use over weeks to months may help fix the issue.