BACKGROUND: Transcutaneous electrical nerve stimulation (TENS) is a non-pharmacological treatment that works by delivering electrical currents via electrodes attached to the skin at the site of pain. It can be an alternative to pharmacological treatments. The mechanism of action of TENS for pain relief is related to the inhibition of the transmission of painful stimuli, release of endogenous opioids, and reduced muscle ischaemia of the uterus. Although it has been used for primary dysmenorrhoea ((PD); period pain or menstrual cramps), evidence of the efficacy and safety of high-frequency TENS, low-frequency TENS, or other treatments for PD is limited. OBJECTIVES: To evaluate the effectiveness and safety of transcutaneous electrical nerve stimulation (TENS) in comparison with placebo, no treatment, and other treatments for primary dysmenorrhoea (PD). SEARCH METHODS: We searched the Gynaecology and Fertility Group's Specialized Register of controlled trials, CENTRAL, MEDLINE, Embase, PsycINFO, AMED, CINAHL, and the Korean and Chinese language databases up to 9 April 2024. We also searched for ongoing trials in trials registries and the reference lists of relevant studies for additional trials. Language restrictions were not applied. SELECTION CRITERIA: We included randomized controlled trials (RCTs) that included women (aged 12 to 49 years) with PD. Included trials compared low-frequency TENS or high-frequency TENS with other TENS, placebo, or other treatment. DATA COLLECTION AND ANALYSIS: Four review authors screened the trials, extracted the data according to the protocol, assessed the risk of bias using RoB 2, and assessed the certainty of evidence for all review comparisons and primary outcomes (i.e. pain relief and adverse effects) using the GRADE approach. MAIN RESULTS: This review replaces the current review, published in 2009. We included 20 RCTs involving 585 randomized women with high-frequency TENS, low-frequency TENS, placebo or no treatment, or other treatment. We included five comparisons: high-frequency TENS versus placebo or no treatment, low-frequency TENS versus placebo or no treatment, high-frequency TENS versus low-frequency TENS, high-frequency TENS versus other treatments, and low-frequency TENS versus other treatments. High-frequency TENS versus placebo or no treatment High-frequency TENS may reduce pain compared with placebo or no treatment (mean difference (MD) -1.39, 95% confidence interval (CI) -2.51 to -0.28; 10 RCTs, 345 women; low-certainty evidence; I = 88%). Two out of three RCTs reported no adverse effects and hence we were unable to estimate the effect of high-frequency TENS on adverse effects. Low-frequency TENS versus placebo or no treatment Low-frequency TENS may reduce pain compared with placebo or no treatment (MD -2.04, 95% CI -2.95 to -1.14; 3 RCTs, 645 women; low-certainty evidence; I = 0%). No trials reported adverse effects for this comparison. High-frequency TENS versus low-frequency TENS It is uncertain whether high-frequency TENS had an effect on pain relief compared with low-frequency TENS (MD 0.89, 95% CI -0.19 to 1.96; 3 RCTs, 54 women; low-certainty evidence; I = 0%). One trial contributed data on adverse effects but no adverse events occurred. High-frequency TENS versus other treatments It is uncertain whether high-frequency TENS had an effect on pain relief compared to acupressure (MD -0.66, 95% CI -1.72 to 0.40; 1 RCT, 18 women; very low-certainty evidence), acetaminophen (paracetamol) (MD -0.98, 95% CI -3.30 to 1.34; 1 RCT, 20 women; very low-certainty evidence), and interferential current therapy (MD -0.03, 95% CI -1.04 to 0.98; 2 RCTs, 62 women; low-certainty evidence; I = 0%). The occurrence of adverse effects may not differ significantly between high-frequency TENS and NSAIDs (OR 12.06, 95% CI 0.26 to 570.62; 2 RCTs, 88 women; low-certainty evidence; I = 78%). Low-frequency TENS versus other treatments It is uncertain whether low-frequency TENS had an effect on pain relief compared with acetaminophen (MD -1.48, 95% CI -3.61 to 0.65; 1 RCT, 20 women; very low-certainty evidence). No trials reported adverse effects for this comparison. AUTHORS' CONCLUSIONS: High-frequency TENS and low-frequency TENS may reduce pain compared with placebo or no treatment. We downgraded the certainty of the evidence because of the risk of bias. Future RCTs should focus more on secondary outcomes of this review (e.g. requirement for additional analgesics, limitation of daily activities, or health-related quality of life) and should be designed to ensure a low risk of bias.