Which meds should I use to treat Irritable Bowel Syndrome?

Since the med stopped working, or you may have another problem. If it was me I would get a new colonoscopy and see if you have any new polyps or any other problems in there.
I found a great notation on the net about IBS. I hope it helps

"Trying Prescription Drugs for Irritable Bowel Syndrome
Remember that while there are a number of different prescription medications available for IBS, their effectiveness does vary greatly from one person to another.

What particular drug (if any) will work best for you is something you'll most likely have to determine through trial and error. If the first medication you try doesn't help much, keep in mind that there are other options available.

You should work in partnership with your doctor to determine which medication best fits your needs. "

http://www.helpforibs.com/books/1stIBS/prescriptions3.asp

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This article also gives you names of drugs out there and what they are used for. They also list two drugs that are due shortly for hitting the market.