Which medication is commonly used as an antidiarrheal?

Bismuth subsalicylate is commonly used as an antidiarrheal due to its ability to decrease the frequency of bowel movements and provide soothing effects on the gastrointestinal tract. It works by several mechanisms, including coating the lining of the intestines, reducing inflammation, and possessing mild antibacterial properties, which can help combat diarrhea caused by certain infections. This medication is effective for treating both mild diarrhea and symptoms related to conditions like traveler's diarrhea and is often found in over-the-counter formulations.

In contrast, metformin is primarily used for managing type 2 diabetes and is not intended for treating diarrhea. Ondansetron is an antiemetic that is effective in preventing nausea and vomiting but does not address diarrhea. Polyethylene glycol is used as a laxative to relieve constipation, not for the management of diarrhea. Thus, bismuth subsalicylate stands out as the appropriate choice for its specific antidiarrheal properties.