One-third of diabetic patients prescribed metformin—the mostly commonly recommended diabetes drug—aren’t taking the medication due to side effects ranging from diarrhea to depression, researchers in the United Kingdom have found.

A team of scientists at the University of Surrey, headed by Andrew McGovern, BMBS, pulled data for more than 1.6 million type 2 diabetes patients from medical databases across the web in an effort to determine the likelihood of diabetics sticking with their suggested medication routine. McGovern and his team published their results this month in Diabetes, Obesity and Metabolism.

After analyzing 48 studies, which compared both oral drugs and injectable therapies, McGovern et al. found that metformin, though the most common of diabetes drugs, was the least likely to be taken on an ongoing basis. While just 70 percent of patients stick to their metformin routines, medications like gliclazide and pioglitazone had higher rates of adherence at 23 percent and 20 percent, respectively, according to the research.

DPP4 inhibitors—a newer class of diabetes drug also known as gliptins—“interestingly” had the highest rates of adherence, the authors wrote, with all but 10 to 20 percent of medication doses skipped. When it came to injectable therapies, McGovern and colleagues found diabetic patients were twice as likely to stop taking GLP1 receptor agonists like exenatide compared with insulin.

The authors postulated these gaps in adherence were due in part to the side effects of metformin, which can be debilitating in rare cases. Mayo Clinic cites anxiety, blurred vision, chills, fast heartbeat, fever, nausea, seizures, nightmares and slurred speech as just a handful of common symptoms; rarer ones spanned from muscle pain to fainting spells to hematuria.

“We have known for a long time that a lot of medications prescribed for chronic diseases never actually get taken,” McGovern said in a release from the University of Surrey. “What this latest research suggests is that patients find some of these medication classes much easier to take than others.”

McGovern said the importance of diabetics taking their prescribed meds “cannot be underestimated.” Not sticking to a routine, he said, can complicate a patient’s condition and lead to kidney damage and eye disease.

“I urge anyone who is struggling to take their medication as prescribed, whether this is because of side effects or because the schedule is too complicated, to discuss this openly with their doctor or nurse,” he said. “Fortunately for type 2 diabetes we have lots of treatment options, and switching to a different medication class which is easier to take could provide an easy way to improve adherence. I would also encourage doctors and nurses to actively ask their patients about medication adherence.”