Over-the-counter medications for heartburn, reflux and morning sickness are at the centre of a class action investigation over alleged links to stomach cancer and kidney damage that could involve an estimated 100,000 Australians.

Patients who have used proton pump inhibitors (PPIs) to treat digestive issues, and subsequently developed gastric cancer or kidney damage, are eligible to join the class action investigation launched by Shine Lawyers last month.

One of those is Donna Johnstone, who claimed the heartburn medication Somac led to her developing kidney failure.

“I was taking Somac regularly. I was never informed of any ill effects or anything from it,” Johnstone said.

“One day, doctors were doing just routine blood tests when they called me. When I got there, he said my kidney function was at 19 per cent and if it got to 15 per cent, I’d have been on dialysis.

“They got me in to see a specialist, and the professor just said, straight up, this is to do with the Somac you’ve been taking.

“I then went to have a biopsy on my kidneys, and that pretty much just confirmed what he already knew, that I had kidney failure and that it was the Somac causing it.”

The inhibitors, commonly taken for digestive symptoms and disorders, work by decreasing the enzymes that line the stomach and cause stomach acid.

This process can increase gastrin levels, which is a hormone produced in the gastrointestinal, urinary tract and nervous system. An increase in gastrin has been linked to the formation of cancer cells.