Rationale
Microdosing psychedelics is the repeated use of small doses of, for example, lysergic acid diethylamide (LSD) and psilocybin, typically for a few weeks. Despite the popular and scientific attention in recent years, and claims by users that it has therapeutic value in affective disorders like depression, little scientific knowledge is available to back this.

Objective
The purpose of this review was to investigate whether there are scientific grounds to state that this practice could be helpful in the treatment of affective disorders, and safe to use repeatedly

Methods
To that end, the literature (PubMed, MedLine) was searched, looking for (controlled) experimental studies with low doses of LSD and/or psilocybin, in healthy volunteers and patient samples. After a selection process and the addition of relevant articles, 14 experimental studies entered this review.

Results
Findings show that both LSD (10–20 mcg) and psilocybin (<1–3 mg) have subtle (positive) effects on cognitive processes (time perception, convergent and divergent thinking) and brain regions involved in affective processes. Besides the pleasant experience, increased anxiety and a cycling pattern of depressive and euphoric mood were also found. With regard to safety, it was demonstrated that low doses are well tolerated (in healthy volunteers) and have no-to-minimal effects on physiological measures. Conclusion While it is yet unclear whether psychedelic microdosing is of therapeutic value for depression, the aforementioned effects on selective processes suggest that low doses of psychedelics could play a role in depression by inducing some kind of cognitive flexibility, which might lead to decreased rumination. While previous studies were conducted mostly in small samples of healthy volunteers, future placebo-controlled clinical trials in depressed patients are required to understand the therapeutic value of microdosing psychedelics, how this differs from therapy using full psychedelic doses, and whether different psychedelics have different effect patterns. The proposed research will give new insights into the potential of future alternative psychiatric treatment forms that are fiercely needed.