In routine services, the useful dose of therapy runs from four to twenty six sessions
A review of 26 studies of therapy as it is actually delivered found the same curve every time: fast early change that flattens. Optimal doses ran from 4 to 26 sessions, and weekly therapy moved faster than less frequent schedules.
Suggestive but preliminary. Not settled. how we score evidence
Caveat on this rating: Dose-response studies in routine care are observational: nobody was randomised to a number of sessions, so the curve describes what happened, not what a fixed dose would do. The review is explicit that chronic and severe disorders are under-studied. The 4 to 26 range is a spread across settings, not a prescription for any one person.
There is a version of therapy that is a standing appointment for life. The research on how long therapy takes does not describe that. It describes a treatment with a dose and a curve.
Significant findings
In 2020 Robinson, Delgadillo and Kellett published a systematic review of the dose-response effect in psychological therapies as they are routinely delivered to adults. Twenty six studies were eligible. The methods varied, from survival analysis to multilevel modelling, but the shape did not. The review found "replicated and consistent support" for a curvilinear relationship between treatment length and outcome: most of the change happens early, and each later session adds less.
The number people want is in the abstract. "Optimal doses of psychotherapy in routine settings range between 4 and 26 sessions (4-6 for low intensity guided self-help)", varying by setting, population and how outcome was measured.
Frequency mattered too. "Weekly therapy appears to accelerate the rate of improvement compared to less frequent schedules."
The other direction
The review names its own limits. The curve had "few exceptions such as eating disorders and severe psychiatric populations". Most of the evidence came from university counselling centres and outpatient clinics treating common mental health problems. For people with chronic and severe disorders the evidence is "scarce and inconclusive". If that is your situation, the 4 to 26 range was not measured on you.
What this does not show
A curve that flattens across everyone does not prove that late sessions do nothing for the people still attending. Other rows in this set explain why: the people who get what they need early tend to leave, so the people left at session twenty are the ones for whom change is slower.
Worth asking
What are we working on, and how will we both know it has changed?
Source
The dose-response effect in routinely delivered psychological therapies: a systematic review — Robinson L, Delgadillo J, Kellett S (2020)
Reputable peer-reviewed journal
Read the source: https://doi.org/10.1080/10503307.2019.1566676
DOI: 10.1080/10503307.2019.1566676
How this was scored
- Study design
- Systematic review of observational studies
- Funding
- Funding not disclosed
- Published in
- Reputable peer-reviewed journal
- Sample size
- not recorded
- Preregistered
- not recorded
- Conflicts disclosed
- not recorded
- Independent of proponent
- not recorded
- Retracted
- No
Read the full scoring rubric, including what it can't tell you.
Published September 24, 2026.
Questions
How strong is the evidence behind this?
veisund rates this source "early signal". Suggestive but preliminary. Not settled. It scores 52 out of 100 on our published rubric. One caveat travels with that badge: Dose-response studies in routine care are observational: nobody was randomised to a number of sessions, so the curve describes what happened, not what a fixed dose would do. The review is explicit that chronic and severe disorders are under-studied. The 4 to 26 range is a spread across settings, not a prescription for any one person. The score is calculated from recorded facts about the source — study design, funding, publication venue, sample size, preregistration — not typed in by an editor.
What is the source for this?
The dose-response effect in routinely delivered psychological therapies: a systematic review — Robinson L, Delgadillo J, Kellett S (2020). Published in: Reputable peer-reviewed journal. DOI: 10.1080/10503307.2019.1566676. The full source is linked on this page so you can read it yourself.
Who paid for this research, and does that matter?
Study design: Systematic review of observational studies. Funding: Funding not disclosed. Independence from the proponent is not recorded. Industry sponsorship is one of the most reliably measured biases in medicine, which is why funding carries real weight in the score rather than sitting in a footnote.
Is this medical advice?
This is information to bring to your prescriber, not medical advice and not a reason to change anything on your own. Nothing here is an instruction to stop or reduce a medication. If you are in crisis, call or text 988.
This is information to bring to your prescriber, not medical advice and not a reason to change anything on your own. Nothing here is an instruction to stop or reduce a medication. If you are in crisis, call or text 988.