A new study has found that indigestion pills taken by millions are actually increasing the risk of stomach cancer by a significant amount.

Scientists are reporting that people who regularly use proton pump inhibitors (PPIs) – common drugs used to treat acid reflux – are twice as likely to develop stomach cancer.

When people took the drugs long-term, the risk rose astronimcally, increasing by five times after a year of taking the drugs to more than eight times after three years of regularly taking the pills.

Research scientists from University College London and the University of Hong Kong theorized that the pills stimulate a hormone called gastrin, which triggers the growth of cancerous cells.

More than five million bottles and packets of PPIs – which include omeprazole and lansoprazole – are prescribed each year in England alone to treat gastroesophageal reflux, a severe form of heartburn. Many people even buy these stomach drugs over the counter at pharmacies without a prescription, or in corner shops and supermarkets.

The drugs are not recommended for long-term use, but doctors fear that because they are so readily available, people may take them without medical supervision for years.

Many now fear the health impacts of the drugs if taken for too long, with new research linking sustained use of PPIs to dementia, heart attacks and kidney problems. New research published in the BMJ journal Gut, involved 63,000 people in Hong Kong. At the beginning of the study, they were treated with antibiotics to kill bacteria called H. pylori, which is linked to stomach cancer.

The researchers did this to rule out the role of bacteria in the development of cancer, increasing their confidence that the PPI drugs were to blame. The patients were then tracked for an average of seven years.

“We found that long-term use of PPIs increased the risk of gastric cancer development,” researchers wrote. “There was a clear dose-response and time-response trend of PPIs uses and gastric cancer risk. Physicians should exercise caution when prescribing long-term PPIs.”

The researchers calculated that this means that for every 10,000 people who take PPIs, roughly eight people a year will develop stomach cancer – four more than if none were taking the pills.