Constant pain and abdominal bloating forced her to re-organize her life around her IBS.
"I turned down lunch dates. I turned down fishing. I love to fly fish," she says.
After nearly a year of trying out different therapies to no avail McMahon finally found relief with a drug called "Rifaximin."
"I did feel really immediate results and kind of jumped right back into my old routine," she says.
Rifaximin is an antibiotic that targets an overgrowth of bacteria in the gut, thought to be one of the causes of some forms of IBS.
Two new large studies from Cedars-Sinai Medical Center in Los Angeles confirmed that theory.
"We are touching a cause of IBS, and that's the first time," says Dr. Mark Pimentel. "We've never really had a drug that truly treats a potential cause of IBS."
Dr. Pimentel led the latest research.
He says all symptoms greatly improved for nearly half of the patients who took the drug for two weeks in the studies.
That relief lasted for at least three months, something not seen with other drugs.
He said it could be useful for the 70-percent of IBS patients suffering from diarrhea, not constipation.
McMahon wanted to share her story because for many the thought of talking about IBS turns their stomach.
"It's not a comfortable topic, so I think the name steers people away from telling even a doctor that their having an issue," she says.
In this case easing the stigma could also help ease symptoms.
The Food and Drug Administration has not approved Rifaximin to treat IBS, even though some doctors have been prescribing it off-label.
Approval could come this year.