Keywords

Microdosing
Psychedelics
Hallucinogen
LSD
Psilocybin
Low dose
Systematic review

1. Introduction

Microdosing is the practice of regularly ingesting very low doses of psychedelic substances, usually for the purpose of improving wellbeing, cognition, mood, or interpersonal processes (Kuypers et al., 2019). Over the past five years, the popularity of microdosing has increased rapidly in Western societies (Cameron et al., 2020, Winstock et al., 2020). Whereas illicit drug use of all kinds has often been considered a taboo topic, microdosing is now positively discussed in mainstream news stories (Leonard, 2015), documentaries (Gleiberman and Gleiberman, 2020), books (Waldman, 2017), movies (Schroeder, 2019) and entertainment television (Nicholson, 2018). After describing what microdosing entails and providing context for the sudden popularity of this practice, this review outlines and summarises scientific findings on the effects of microdosing from both the first and current waves of psychedelic research. We draw out the most robust findings to date, examine the methodological quality of the included studies, and discuss patterns across the literature that may shed light on the possible actions and effects of microdosing. We conclude with several open questions for the field, and provide a list of recommendations for a robust science of microdosing.

1.1. What is microdosing?

Microdosing can refer to the ingestion of a wide range of psychedelic substances at very low doses: lysergic acid diethylamide (LSD) and psilocybin are the most common, but people also report microdosing with mescaline, dimethyltryptamine (DMT), amphetamines, Salvia divinorum and other research chemicals (Polito and Stevenson, 2019, Rosenbaum et al., 2020). Critically, unlike other forms of psychedelic use, microdosers usually consume these substances regularly or semi-regularly for prolonged periods of time (for example, a common schedule is to dose every 3 days; Rosenbaum et al., 2020).

1.2. How much is a microdose?

The precise quantity that constitutes a microdose is difficult to define, and to date there have been no consistently accepted criteria amongst researchers. The most commonly reported definition is that a microdose is a dose between approximately one tenth and one twentieth of a typical recreational dose, although this range is uncomfortably imprecise for scientific purposes. There are perhaps three key reasons for uncertainty in defining dosing criteria. First, as microdosing typically involves taking unregulated substances, users cannot be confident about the identity of their drugs, or quantities of the active constituents they contain.

Second, there is considerable variation in pharmacological and subjective effects within and across substances, and also across individual responses to a given substance. That is, it is difficult to establish equivalent dose ranges for different classes of drug (e.g., LSD vs. psilocybin), for variants of a given class (e.g., different species of psilocybin-containing mushrooms), for different methods of preparation (e.g., identical mushrooms dried or fresh), or for different people (e.g., individual differences in subjective effects to an identical dose and substance can vary widely).

Third, there is no consensus regarding the subjective effects (or lack of effects) that should be associated with microdosing. In popular reports and guides, microdosing is often referred to as ‘sub-perceptual’, meaning that users should take a dose so low that they cannot identify any drug effects (e.g., Leonard, 2015). Many microdosers claim anecdotally that this is the case (i.e., that they notice no effects of microdosing). Yet, in qualitative studies, participants often describe alterations of consciousness (Andersson and Kjellgren, 2019) and in lab-based studies, participants frequently report some acute effects following ingestion of microdoses (see 3.3.5). This suggests that individuals who are microdosing often have insight into subtle subjective changes. Considering this, it may be that the effects of microdosing are not truly sub-perceptual, and instead may better be described as sub-hallucinogenic (e.g., Anderson et al., 2019a; Cameron et al., 2020; Petranker et al., 2020; Rosenbaum et al., 2020).

As a consequence of these difficulties, not all microdosing studies have specified explicit dose ranges. Based on doses and associated subjective effects that have been reported, we summarise plausible ranges for microdosing various substances in Table 1.

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