Internal Family Systems therapy have not been subjected to large rigorous clinical trials. Most of the studies were pilots with no confirmatory studies as yet. I am just repeating the IFS website which best summarises the three existing studies in rheumatoid arthritis, PTSD and depression. I could not find any decent trials in my preliminary search.
A pilot study of 13 patients with PTSD and comorbidities found a 92% improvement after completing 16 sessions.
A randomised trial of 79 patients with rheumatoid arthritis, the 39 patients receiving IFS had reduced pain and depressive symptoms, while improving physical function and self-compassion.
A study of female college students did not find any difference in the 17 students treated with IFS compared with the 15 students treated with usual care of IPT or CBT, which supports IFS's efficacy, but the therapists in the IFS arm mainly only had 1 year of IFS training and experience.