Can Antibiotics Cause Depression? What the Research Actually Shows
The connection between antibiotics and depression is real, documented, and almost never mentioned when the prescription is written. Here is what the evidence shows and what it means for how we think about mental health.
In 1951, a new antibiotic called iproniazid was being trialled on tuberculosis patients in a New York ward. It worked on the tuberculosis. It also did something nobody expected. Patients who had been listless and despairing began laughing in the hallways. Some danced. The drug was producing what could only be described as euphoria in people who had been ill and depressed for years. Researchers subsequently identified that iproniazid was increasing serotonin levels in the brain. They had stumbled onto the mechanism that would underpin every antidepressant developed for the next seven decades. The first antidepressant was an antibiotic. The field noticed the serotonin and moved on. What it did not ask was what the antibiotic itself had done to get there.
What the gut is actually doing
Approximately ninety percent of the body’s serotonin is produced in the gut. Not in the brain. In the gut, specifically in enterochromaffin cells lining the gastrointestinal tract, with production regulated largely by the gut microbiome, the ecosystem of bacteria, fungi, and other microorganisms inhabiting the digestive system. The gut also contains between 200 and 600 million neurons, more than the spinal cord, and is connected to the brain via the vagus nerve in a continuous bidirectional communication pathway researchers now call the gut-brain axis.
The gut microbiome does not merely produce serotonin. It produces the precursors for dopamine and GABA, regulates the body’s stress response system, governs inflammatory signaling throughout the body, and communicates continuously with the brain through the vagus nerve. Gut dysbiosis, the disruption of this ecosystem, does not stay in the gut. It propagates upward through every pathway available to it. What happens in the gut is felt in the mind.
Antibiotics are, by definition, designed to kill bacteria. They do not distinguish between the bacteria causing an infection and the bacteria maintaining the ecosystem on which mood, cognition, and nervous system regulation depend. A broad-spectrum antibiotic course does not pause the gut microbiome. It depletes it.
What the research shows
In 2015, Ido Lurie and colleagues at the University of Pennsylvania published a nested case-control study in the Journal of Clinical Psychiatry using a large population-based medical record database from the United Kingdom covering 1995 to 2013. The study included 202,974 patients with depression and 803,961 matched controls. The finding was unambiguous: a single course of antibiotics was associated with a measurably higher risk of depression across all antibiotic groups. A single course of penicillin was associated with a 23% increased risk. A single course of quinolones was associated with a 25% increased risk. With two to five courses of penicillin, the risk rose to 40%. With more than five courses, it rose to 56%. The risk increased with both the number of courses and the number of different antibiotic agents used. The association held across antibiotic classes. The risk of depression, the study found, could persist for months to years after the exposure.
A 2025 study from VA Iowa City researchers examined nearly 900,000 veterans who received antibacterial prescriptions and tracked depression incidence for six months before and after. Antibiotic use was associated with a small but statistically significant increase in depression risk, with the elevation highest in the eight weeks immediately following the prescription. Broad-spectrum antibiotics conferred the highest risk. Published in the Journal of Clinical Psychopharmacology.
A 2022 systematic review in the Journal of Psychosomatic Research synthesized available evidence and concluded that studies from the United Kingdom and Sweden indicated antibiotic use increases the risk of depression by at least twenty percent, with risk rising with the number of courses and agents used, and depression risk potentially persisting for months to years following exposure.
These are not small or marginal studies. The Lurie study alone included over a million participants. The association is consistent, it scales with dose, and it persists over time. None of this appears on the information sheet handed to patients with an antibiotic prescription.
The irony at the center of this
SSRIs, selective serotonin reuptake inhibitors, are the pharmaceutical response to depression. They are prescribed to approximately 13 percent of Americans over the age of twelve. Their mechanism is generally described as increasing serotonin availability in the brain. What is not generally described is that SSRIs themselves have antibiotic properties. They inhibit bacterial serotonin transporters in the gut, alter the composition of the gut microbiome, and reduce microbial diversity in ways that compound the dysbiosis already associated with depression. The drug prescribed for a condition associated with gut dysbiosis causes further gut dysbiosis. Research from the University of Ottawa and Beijing Institute of Lifeomics, summarized in 2025, found that SSRIs, SNRIs, and tricyclic antidepressants all altered gut microbiome composition and function.
People with depression are already known to have markedly different gut microbiome profiles from people without depression. The medication designed to treat the condition uses a mechanism that further disrupts the system thought to be contributing to the condition. This is not a reason to stop antidepressants without medical guidance, and people on SSRIs should not alter their treatment without speaking to a physician. It is a reason to ask why the gut microbiome is not part of the standard conversation about both diagnosis and treatment.
What the early evidence suggests about recovery
The same research that documents the connection between gut dysbiosis and depression has begun to document the reverse. Probiotic interventions in clinical studies have shown meaningful reductions in depression scores in both healthy individuals and patients with major depressive disorder. Dietary changes that support microbial diversity, fermented foods, fiber-rich diets, reduced processed food consumption, show measurable effects on mood and anxiety in randomized controlled trials. Fecal microbiota transplantation, still experimental, has produced striking improvements in mood in some depression patients.
None of this is simple or immediate. The gut microbiome is a complex ecosystem that takes time to disrupt and time to recover. But the direction of the evidence is consistent: the gut is a primary site of mental health, not a secondary one, and interventions targeting the gut produce mental health outcomes. The brain is downstream of the gut in ways that the pharmaceutical model of depression has not yet fully incorporated. Which means that the question worth asking when the mood will not lift is not only what is happening in the mind. It is what is happening in the body below it.
The transfer that nobody names
Antibiotics are genuinely lifesaving. They are also prescribed at a scale and frequency that treats the gut microbiome as collateral damage rather than critical infrastructure. The cost of that treatment decision does not appear on the ledger of the prescribing physician, the pharmaceutical manufacturer, or the health system. It appears as a subsequent depression diagnosis, a subsequent antidepressant prescription, a subsequent chronic condition managed pharmacologically over years. Each step in that sequence generates revenue. The original disruption is never named as a cause.
The next article documents a compound that has been doing the same thing to the same system, at population scale, through the food supply, without a prescription and without a label: Glyphosate Is an Antibiotic. Read the full account of the systems producing this harm and what they cost your body in The Political Gut.
Features
Glyphosate, the gut, and the mental health crisis nobody is connecting / How to regulate your nervous system / Why glyphosate is not a food issue
- Lurie I, Yang YX, Haynes K, Mamtani R, Boursi B (2015). Antibiotic exposure and the risk for depression, anxiety, or psychosis: a nested case-control study. Journal of Clinical Psychiatry. 76(11):1522-1528. DOI:10.4088/JCP.15m09961. PMID:26580313.
- VA Iowa City (2025). Antibiotics may increase risk of depression. Journal of Clinical Psychopharmacology. April 2025. Study of nearly 900,000 veterans.
- Turna J, et al. (2022). Antibiotic use and the development of depression: A systematic review. Journal of Psychosomatic Research. DOI:10.1016/j.jpsychores.2022.110902.
- Yano JM, et al. (2015). Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell. 161(2):264-276. 90% of serotonin produced in gut.
- Dinan K, Dinan T (2022). Antibiotics and mental health: The good, the bad and the ugly. Journal of Internal Medicine. DOI:10.1111/joim.13543.
- Foster JA, Neufeld KA (2013). Gut-brain axis: How the microbiome influences anxiety and depression. Trends in Neurosciences. 36(5):305-312.
- University of Ottawa / Beijing Institute of Lifeomics (2025), summarized by Quadram Institute. SSRIs, SNRIs, and tricyclic antidepressants alter gut microbiome composition and function.
- Psychology Today / Anderson SC (2022). The Serotonin Hypothesis and the Gut-Brain Axis. Documents iproniazid as first antidepressant discovered accidentally as antibiotic for tuberculosis, 1951.
- Note: People taking antidepressants or antibiotics should not alter their medication regimen without consulting their physician. The evidence presented is epidemiological and mechanistic; individual circumstances vary.