MiraLAX (polyethylene glycol 3350) is used as a laxative to treat occasional constipation or irregular bowel movements. MiraLAX is available over-the-counter and in generic form. Common side effects of MiraLAX include nausea, abdominal cramping, bloating, upset stomach, gas, dizziness, or sweating.The usual dose of MiraLAX is 17 grams (about 1 heaping tablespoon) of powder per day (or as directed by physician) in 4 to 8 ounces of water.et emergency medical help if you have any of these signs of an allergic reaction: hives; difficulty breathing; swelling of your face, lips, tongue, or throat. Stop taking polyethylene glycol 3350 and call your doctor at once if you have any of these serious side effects: 1  severe or bloody diarrhea;