Is it all about the dose?

How long should psychotherapy last? This is a question many ask when they first seek help for their mental health problems. Is one session enough, or should one invest several months or even years to achieve lasting results? The answer, as is so often the case in psychotherapy, is not straightforward. While some people benefit after just a few sessions, others may need longer therapies. This post will focus on what psychotherapy research has to contribute to this question, and what factors influence what the optimal duration of treatment should be.

What does the research say?

The "dose" of psychotherapy is commonly conceptualized in research as the number of sessions completed. The question of the optimal number of sessions - and whether such a number exists at all - is still being debated. There have been attempts to answer it by having researchers look at many treatment courses in which the patient's well-being was regularly evaluated. This way, two competing theories on this have emerged in recent years.

The first, older theory is the "dose-response model", popularized by psychotherapy researcher Ken Howard. Based on data from over 2400 patients, he found that about 50% of patients measurably improve after eight sessions. After 26 sessions, 75% of patients improved. However, patients who were diagnosed with a more severe (“borderline-psychotic”) disorder took much longer to show improvement: 75% of these patients improved after 52 sessions. Howard's study led him to conclude two things. First: the likelihood of showing measurable success increases with the increasing duration of therapy. While the majority of patients achieve this goal after quite a few sessions, there is a small percentage of severely distressed individuals who require long therapies. Second: the longer the therapy, the fewer patients benefit from additional sessions.

Howard’s dose-response model suggests that the longer therapies get, the fewer patients profit from additional sessions.

The second, more recent theory is called the “good-enough level model”. Ten years after Howard’s analysis, psychotherapy researcher Michael Barkham suspected that the “dose-response” pattern Howard had proposed arose from lumping patients with long and short therapies together. Instead, his theory assumes that patients stay in therapy until they reach the improvement they need. Typically, at this point, the patients and the therapist agree to end the therapy. In a landmark study, Barkham looked at a large database of therapy courses that had such a planned ending and found that the number of sessions was irrelevant to the success rate. He also found that patients who reported more anxiety, depression and other problems before starting therapy needed more sessions. This supports the idea that patients stay in therapy as long as they need. More serious problems need more time to resolve.

We now have two competing theories. The dose-response model states that the length of therapy improves the odds of success, whereas, in the Good Enough Level model, success affects the length of therapy. But which model is correct?

A recent review of the available studies including more than 114.000 patients concluded that both have their strengths and weaknesses. Based on the available data, the researchers found that timing is indeed key in treatment. Longer doesn't always mean better and patients stay in therapy as long as they achieve their goals. However, when looking at therapies that end prematurely, longer treatments show better outcomes. This is probably because they are closer to the agreed-upon goal of treatment. Severe cases may require more sessions, but most patients improve in under 20.

Which factors determine therapy length?

How long therapy can last usually depends on how many sessions can be paid for. In many European countries, psychotherapy is partially or fully covered by statutory health insurance. This is the case, for example, in Germany, which happens to be my home country. In Germany, insurances cover up to 80 sessions of cognitive-behavioral therapy (CBT) and even up to 300 sessions of psychoanalytic therapy.

But even under these conditions, the science on the optimal number of sessions seems to be right. A recent analysis of health records from more than 460.000 patients showed that 77% of psychotherapies end after 24 sessions or less. This is pretty close to Howard’s estimate of 26 sessions and only slightly above the most recent number of 20. Since psychotherapists in Germany are not required to measure their therapy outcomes with standardized tests, it is unclear how many of these therapies ended with meaningful effects. It is also unlikely that all of the therapies had planned endings. A study financed by a large German insurance company found that roughly half of the patients recover from their problems and 70% change meaningfully while in therapy, which is on par with studies from other countries.

The length of therapy also depends on the theoretical orientation. While cognitive behavioral therapy (CBT) takes a structured approach that is easy to accomplish within certain time limits, psychodynamic therapy often works with information that first emerges from the relationship between patient and therapist. This process cannot be rushed so easily. This also shows in the analysis of German patients mentioned earlier. While the therapies of roughly 80% of CBT patients ended after 24 sessions, this was the case only in 35% of patients in psychoanalytic therapy and 50% of patients in psychodynamic therapy.

Comparing the effectiveness of different therapy methods would require a separate post. However, if one considers the studies on the ideal duration of psychotherapy to be valid, it is possible that most therapy methods are successful in the end - but they may “get there” at different speeds.

Is it really about the number of sessions?

Admittedly, the number of sessions is a pretty crude measure of how "much" therapy someone has experienced. You can have four very intense, exhausting sessions, and you can barely budge years of therapy. A clinic in the Netherlands that specializes in the treatment of post-traumatic stress disorder (PTSD) offers psychotherapy that is completed after eight full days of exposure and EMDR treatment. In between, the patients are educated about trauma and do sports. After this program, 74% of patients no longer fulfill the criteria for PTSD, even though the patients had severe PTSD that was often related to childhood abuse. It is plausible that such intensive programs are so effective because they are much more vividly anchored in the memory.

Bottom line: patients can expect to get better from psychotherapy after about 20 sessions, but this depends on the approach their therapist uses. Research, on the other hand, needs to find a better measure to quantify the “dose” of therapy so that their theoretical models accommodate newly developed, intensive treatment approaches.