Which of the following is a common drug used as a laxative?

Polyethylene glycol is a widely used osmotic laxative that works by drawing water into the intestines, which helps to soften the stool and promote bowel movements. It is commonly prescribed for patients experiencing constipation or preparing for certain medical procedures that require bowel cleansing. This mechanism increases the water content in the stool, making it easier to pass.

The other options do not serve the primary function of a laxative. Loperamide is an antidiarrheal medication, which slows down intestinal motility to reduce the frequency of bowel movements. Glipizide is an oral hypoglycemic agent used to control blood sugar levels in patients with diabetes, and ondansetron is an antiemetic used to prevent nausea and vomiting, particularly related to chemotherapy or surgery. These medications have distinct therapeutic uses that do not include treating constipation.