More than 50% of Americans have been prescribed narcotic painkillers, according to a Truven Health Analytics-NPR Health poll, and more than one third of respondents were concerned about painkillers’ addiction risk, effectiveness and side effects.

The concern towards prescription painkillers differed between those who have been prescribed narcotic pain-relievers and those who have not. Just over 35% of respondents who have been prescribed these drugs said they have concerns – a number that has held steady since 2011.

This trend didn’t hold true for people who haven’t been prescribed narcotic painkillers, according to the data. In 2011, nearly 30% of respondents who have not been prescribed the drugs said they were worried about potential addictive side effects. However, that number grew in 2016 to 46% of respondents.

The most cited reasons for respondents’ concerns were fear of addiction and side effects. For those who have been prescribed the painkillers, fear of side effects ranked higher than fear of addiction. The opposite was true for those who have not been prescribed the drugs by their doctor.

“This data shows that people are becoming more aware of risks associated with narcotic painkillers,” Ronald Ozminkowski, VP of cognitive analytics for the value-based care biz at IBM Watson Health, said in prepared remarks. “What’s most fascinating, however, is that consumer concerns about side effects, addiction and long-term impact associated with the drugs are most pronounced among those who have never been prescribed narcotic painkillers. That exposes an educational challenge caregivers will need to address when prescribing these drugs.”

The results from the survey included data from 3,000 participants surveyed from Nov. 1 to Nov. 15 last year. The researchers added that the margin of error for each survey year is +/- 1.8 percentage points.

Truven Health Analytics is a part of the IBM Watson Health business.