In this study, researchers evaluated the benefits and harms of CES for adult patients with chronic painful conditions, depression, anxiety, and insomnia. Several databases from inception to 10 October 2017 without language restrictions and references from experts, prior reviews, and manufacturers. Randomized controlled trials of CES versus usual care or sham CES that reported pain, depression, anxiety, or sleep outcomes in any language.

Single-reviewer extraction checked by another; dual independent quality assessment; strength-of-evidence grading by the first author with subsequent group discussion. Twenty-eight articles from 26 randomized trials met eligibility criteria. The 2 trials that compared CES with usual care were small, and neither reported a statistically significant benefit in pain or anxiety outcomes for patients with fibromyalgia or anxiety, respectively.

Fourteen trials with sham or placebo controls involving patients with painful conditions, such as headache, neuromuscular pain, or musculoskeletal pain, had conflicting results. Four trials done more than 40 years ago and 1 from 2014 provided low-strength evidence of a possible modest benefit compared with sham treatments in patients with anxiety and depression.

Trials in patients with insomnia (n = 2), insomnia and anxiety (n = 1), or depression (n = 3) had inconclusive or conflicting results. Low-strength evidence suggested that CES does not cause serious side effects. Most trials had small sample sizes and short durations; all had high risk of bias due to inadequate blinding.

Evidence is insufficient that CES has clinically important effects on fibromyalgia, headache, neuromusculoskeletal pain, degenerative joint pain, depression, or insomnia; low-strength evidence suggests modest benefit in patients with anxiety and depression.