Background: In recent years there has been growing media attention on microdosing psychedelics (e.g., LSD, psilocybin). This refers to people routinely taking small doses of psychedelic substances to improve mental health and wellbeing, or to enhance cognitive performance. Research evidence is currently limited. This paper examines microdosing motivations, dosing practices, perceived short-term benefits, unwanted effects, and harm reduction practices. Methods: An international online survey was conducted in 2018 examining people's experiences of using psychedelics. Eligible participants were aged 16 years or older, had used psychedelics and could comprehend written English. This paper focuses on 525 participants who were microdosing psychedelics at the time of the survey. Results: Participants were primarily motivated to microdose to improve mental health (40%), for personal development (31%) and cognitive enhancement (18%). Most were microdosing with psilocybin (55%) or LSD/1PLSD (48%). Principal components analysis generated three factors examining perceived short-term benefits of microdosing: improved mood and anxiety, enhanced connection to others and environment, and cognitive enhancement; and three factors examining negative and potentially unwanted effects: stronger-than-expected psychedelic effects, anxiety-related effects, and physical adverse effects. Most participants (78%) reported at least one harm reduction practice they routinely performed while microdosing. Conclusion: Our findings suggest that people microdosing are commonly doing so as a self-managed therapy for mental health, either as an alternative or adjunct to conventional treatments. This is despite psychedelics remaining prohibited substances in most jurisdictions. Recent findings from clinical trials with standard psychedelic doses for depression and anxiety suggest that a neurobiological effect beyond placebo is not unreasonable. Randomised controlled trials are needed, complemented by mixed methods social science research and the development of novel resources on microdosing harm reduction.