Mild electric shocks supplied by a portable device, a process called TENS, have been used for years to treat chronic low-back pain.

Problem is, it doesn't work, a new study concludes.

Anyone currently getting transcutaneous electric nerve stimulation (TENS) treatments should consult with their doctor about the new findings, researchers said.

"The strongest evidence showed that there is no benefit for people using TENS for chronic low-back pain," said Dr. Richard M. Dubinsky of the Kansas University Medical Center.

However, there is good evidence showing TENS can be effective in treating diabetic nerve pain, the researchers said. More research is needed to see if that's true, they said.

Past research on TENS has produced conflicting results. In the new study, the researchers reviewed all studies involving low-back pain lasting three months or longer. Acute low-back pain was not studied. The evidence to date show that TENS does not help with chronic low-back pain, they conclude.

However, there is good evidence showing TENS can be effective in treating diabetic nerve pain, the researchers said.

One longstanding problem with TENS: Scientists don't know how it can work. One theory is that nerves can only carry one signal at a time, and the electrical stimulation may confuse the brain and block the real pain signal from getting through.

About 75 million U.S. adults experience some sort of chronic pain, with low-back pain among the most common complaints.

Yet scientists admit they don't fully understand how pain works or why we experience it. Research suggests that among the best remedies for chronic pain — after consultation with a physician — is exercise.