Abstract: 29 RCTs, with eight weeks to five years follow-up, met our inclusion criteria; four were excluded from the primary meta-analysis because of inadequate outcome data. The remaining 25 trials assessed 1,198 participants, but adequate randomisation was confirmed in only seven trials and concealment of allocation in only one. Only one trial reported deaths, heart attacks and strokes (one of each). Meta-analysis indicated that relaxation resulted in small, statistically significant reductions in SBP (mean difference: -5.5 mmHg, 95% CI: -8.2 to -2.8, I2 =72%) and DBP (mean difference: -3.5 mmHg, 95% CI: -5.3 to -1.6, I2 =75%) compared to control. The substantial heterogeneity between trials was not explained by duration of follow-up, type of control, type of relaxation therapy or baseline blood pressure. The nine trials that reported blinding of outcome assessors found a non-significant net reduction in blood pressure (SBP mean difference: -3.2 mmHg, 95% CI: -7.7 to 1.4, I2 =69%) associated with relaxation. The 15 trials comparing relaxation with sham therapy likewise found a non-significant reduction in blood pressure (SBP mean difference: -3.5 mmHg, 95% CI: -7.1 to 0.2, I2 =63%). In view of the poor quality of included trials and unexplained variation between trials, the evidence in favour of causal association between relaxation and blood pressure reduction is weak. Some of the apparent benefit of relaxation was probably due to aspects of treatment unrelated to relaxation.

Plain-language summary: Our review pooled findings from 1,198 people with blood pressure over 140/85 mmHg who were enrolled in 25 randomised controlled trials. These trials compared the effect of relaxation either with no treatment or with a dummy treatment which wasn't expected to reduce blood pressure. Overall, relaxation reduced blood pressure by a small amount: the average reduction was 5/3 mmHg, but might be anywhere between 8/5 mmHg and 3/2 mmHg. Different trials gave different − sometimes inconsistent − results. Many of the trials were not well designed or conducted. In the good quality trials, relaxation resulted in smaller average reductions in blood pressure and the results could even be consistent with an average increase in blood pressure. Even when all the trials were put together, the combined group of all the people in all the trials wasn't large enough and the trials didn't run for long enough to tell us whether relaxation could reduce the risk of death, heart attack or stroke. Few people reported side-effects of relaxation and, on average, people were just as likely to report side-effects of the comparison treatment. Different types of relaxation were taught in different trials. It was difficult to disentangle their effects, especially as many trials used a combination of methods. Overall, we found no evidence that autogenic training was effective. Progressive muscle relaxation, cognitive/behavioural therapies and biofeedback seemed to be more likely to reduce blood pressure. However, some of the reduction in blood pressure was almost certainly due to aspects of treatment that were not related to relaxation, such as frequent contact with professionals who were trying to help.