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The Microbiota-Gut-Brain Axis in Mental Health and Medication Response: Parsing Directionality and Causality

Bastiaanssen TFS, Cryan JF (2021) Int J Neuropsychopharmacol  24(3) 216-220. doi: 10.1093/ijnp/pyaa088. 

Web URL: Read this and related abstracts via PubMed here. Free full text of this article is available onlne


There is increasing evidence for the role of the microbiome in various mental health disorders. Moreover, there has been a growing understanding of the importance of the microbiome in mediating both the efficacy and side effects of various medications, including psychotropics.

In this issue, Tomizawa and colleagues report on the effect of psychotropic drugs on the gut microbiome of 40 patients with depression and/or anxiety disorders.

In their longitudinal cohort, the authors find that antipsychotics, but not anxiolytics, decrease microbiome alpha diversity. They further find that antipsychotics dosage was negatively correlated with alpha diversity in these patients. The health consequences of these microbiome alterations remain to be fully understood.

In this commentary, we will discuss such findings through the lens of several recent studies on the microbiota-gut-brain axis. We also use the paper as a backdrop to discuss directionality and, by extension, causality in relation to microbiota-gut-brain signaling.


Only in the last 10-15 years has evidence been emerging to show that our gut microbiota appear to play a fundamental role in almost every aspect of health and disease - including brain development and functioning, and therefore mental health and welbeing.

The so-called microbiota-gut-brain axis is highly complex, involving multiple signalling pathways that work in both directions, i.e. from gut to brain, and from brain to gut.

Research has shown that many common medications in addition to antibiotics can disrupt gut microbial balance and diversity - including non-steroidal anti-inflammatories, drugs used to manage diabetes or acid reflux, and some psychiatric medications. See: 

This article reviews recent evidence on how medications for mental health disorders - collectively known as 'psychotropics' - may influence the gut microbiota, and conversely, how the balance of gut microbes may influence the action of these medications.

For more information on the links between gut and brain health, see also: