Optimal and Minimal Doses of Acceptance and Commitment Therapy to Improve Psychological Distress: A Meta-Analysis, Meta-Regression, and Subgroup Analysis of Controlled Trials.

Optimal doses of psychotherapy prevent overtreatment and help mental health service managers better allocate resources and tailor treatment to fit individual needs. This meta-analysis originally aimed to evaluate the optimal and minimal doses of acceptance and commitment therapy (ACT) to improve overall psychological distress (anxiety, depression, and stress). PubMed, Scopus, and PsycNET databases were systematically searched to discover eligible studies until October 2024. More manual searches were conducted using Google Scholar. Due to heterogeneity, standard mean differences (Cohen's d) were estimated using the random-effects method. Meta-regression and subgroup analyses were used to explore some potential moderators. Analysis of data from 91 studies (6,639 people) from 20 different regions showed that ACT moderately (d = -0.46, p < .001) reduced psychological distress with stable improvements (d = -0.31, p < .001) over time. Meta-regression analysis indicated that the pooled estimates were influenced by seven moderators, including age, gender, control type, population type, geographic region, risk of bias, and study year. Additionally, psychological distress was significantly reduced in 9 of the 17 treatment doses studied, with 8 sessions identified as optimal and 4 sessions as minimal. We conclude that the improvements in the outcome across optimal and minimal doses of ACT are remarkable and relatively stable over time. Evaluating optimal doses of ACT for other psychological outcomes will be the target of future meta-analyses.