Which conventional prescription medications are used to treat ibs?

Meds. Antispasmodics include bentyl, levsin, and librax. Meds for visceral hypersensitivity include tca's like Elavil and ssri's such as celexa. Meds for constipation include Amitiza (lubiprostone) and linaclotide (on the market soon). Probiotics. Antibiotics such as Flagyl and rifaxamin can be helpful. And even Mesalamine agents have been beneficial in case reports.
I prefer None. I have never prescribed drugs for so called ibs. You have to change some bad habits like eating at regular times daily, same quantities, moderate fiber intake and most important 80-100 ounces of caffeine free/alcohol free fluids. Over time possibly 1-3 months your symptoms will resolve, but you must maintain this regimen of retraining your bowel.
Levsin, (hyoscyamine) bentyl. The most common prescription medications for IBS are Levsin (hyoscamine), Bentyl and to a lesser extent librax (clinidium and chlordiazepoxide). Amitza is fda approved for the use in constipation=predominant ibs.