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May 11, 2026 - By Lorna Fernandes - Interest in adolescent mental health is growing as studies highlight increased stress, anxiety, and depression in this age group. Teens are facing increased biological, social, and environmental pressure to manage stressors while their bodies and brains are still developing. According to the National Institute of Mental Health, 1 in 5 adolescents ages 12–17 have had at least one major depressive episode in the past year — an increase from 1 in 9 a decade ago.
As researchers explore new ways to treat mental health conditions in teens beyond traditional therapy and medications, one area of growing interest is the connection between the gut and the brain, known as the gut-brain axis. Related research on adults has shown a connection between the brain and gut bacteria that influences mood through inflammation and brain chemicals like serotonin. This begs the question: Could treating gut bacteria in teens be effective in treating their depression?
Cherry Leung, PhD, RN, associate professor of community health systems with the UC San Francisco School of Nursing, studies the impact of probiotics to supplement prescription medication like antidepressants on adolescents. Her research is supported by National Institutes of Health (NIH) funding.
What is the gut-brain axis, and why is it important for mental health?
The gut-brain axis refers to the communication network between the digestive system and the brain. Research shows that bacteria in the gut can send signals to the brain through multiple pathways, including hormones, the immune system, and the nervous system — particularly the vagus nerve. These signals can influence mood, stress responses, and overall well-being.
The communication is bidirectional, meaning the brain can affect the gut through the same pathways. For example, feeling nervous or stressed can alter digestion and lead to symptoms like nausea or stomach discomfort.
How can probiotics help reduce depressive symptoms?
Probiotics are “good bacteria” that can support gut health by improving microbial balance in the digestive system. Studies suggest that a healthy gut microbiome (the community of bacteria in the gut) can influence mental health in adults by reducing inflammation, regulating stress-related hormones, and affecting neurotransmitters such as serotonin.
Serotonin helps regulate mood, sleep, appetite, and digestion. It contributes to feelings of well-being and happiness, and disruptions in serotonin signaling have been linked to depression and anxiety. Many patients with depression are prescribed Selective Serotonin Reuptake Inhibitors (SSRIs), which work by increasing the availability of serotonin in the brain.
What’s the focus of your work?
My research specifically focuses on how the gut microbiome influences inflammation and stress-response pathways in adolescents with depression. I study how specific microrganisms are structured, how they behave during inflammation, and how the microbiome may shape the body’s physiological response to stress. These biological processes can help explain why changes in the gut microbiome are linked to mental health outcomes, including depression.
Probiotics, including beneficial bacteria such as lactobacillus, are an example of one approach that may help regulate inflammation by interacting with the immune system and supporting a healthier gut environment. Lactobacillus is naturally present in the gut and is also found in fermented foods.
If the gut becomes healthier, it may send balanced signals to the brain through the gut-brain axis, which could improve mood and overall emotional well-being. 
UCSF Assistant Professor of Community Health Systems Cherry Leung, PhD at UCSF’s Parnassus campus. Photo by Noah Berger
Why are adolescents a key group for studying probiotics and mental health?
Teenagers are still developing, both in their brains and in their gut microbiome. Because of this, adolescence may be a period when the gut-brain axis is more adaptable, and treatments like probiotics could have different or potentially stronger effects. However, more research is needed to understand how adolescents respond compared to adults.
Are probiotics used instead of antidepressants in adolescents?
At this time, we are investigating probiotics as an add-on (or adjunct) treatment, not a replacement for prescription medication. Teens in our studies are already taking antidepressants, and we are adding probiotics to see if they can provide additional benefit. The goal is to improve results and outcomes alongside standard treatments, not replace them.
Can families start using over-the-counter probiotics for teen mental health?
Probiotics are widely available and generally considered safe for healthy individuals, but there isn’t yet enough evidence to recommend a specific type or brand for improving mental health. Some clinicians may recommend them since they usually don’t cause harm, but they are not a proven treatment for depression yet. Probiotics are regulated as dietary supplements, not as medications, which means they are not FDA-approved to treat or prevent mental health conditions.
You’ve launched a new study?
We are studying whether probiotics can help balance the bacteria in the gut and influence the brain regions associated with depression, like the hippocampus (which plays a role in memory and emotion) and precuneus (which is involved in self-reflection and mood-related processing).
We are currently recruiting and enrolling 77 adolescents here at the Mission Bay campus for eight weeks, which is a standard duration used to evaluate changes in mood and treatment response.
If probiotics can affect the areas of the brain linked to depression, they may help improve treatment response in adolescents. Antidepressants can have side-effects that are difficult for some individuals to tolerate, so my goal is to determine whether probiotics can be used alongside antidepressants to improve outcomes and potentially reduce side-effects. Ultimately, this work may help inform more personalized mental health treatment in the future.
Source: UCSF
