What happen if I take "polyethylene glycol 3350". For a month?

Many use it longer. You should find out why you have chronic constipation first such as ruling out thyroid disease etc..If no underlying disease, try the natural way such as fibers, roughage, and adequate fluid daily. This med is relatively safe to use and many patients use it for months to yrs. Like any other med, use smallest effective dose for shortest possible time. Consult doc. Good luck.