We know that kissing can strengthen emotional bonds, lower stress, and even burn a few calories, but new research suggests it might also do something far less expected: influence your mental health.
A groundbreaking 2025 study published in Exploratory Research and Hypothesis in Medicine1 reveals that depression and anxiety symptoms may be transmissible between partners through the oral microbiome, especially via intimate behaviours like kissing. It’s a scientific twist on the phrase “you are who you kiss.”
The Study: Microbes, Marriage & Mental Health
In the study, researchers followed 268 newlywed couples over six months, tracking their mental health, sleep quality, and oral microbiota, the communities of bacteria living in the mouth.
Their findings were striking; when one partner experienced symptoms of anxiety, depression, or sleep disturbances, the other partner often began to develop similar symptoms.
The connection? Changes in the healthy partner’s oral microbiome, which began to resemble that of the affected partner. The researchers observed notable increases in bacteria such as Clostridia, Veillonella, Lachnospiraceae, and Bacillus, organisms previously associated with mood imbalance and inflammation. These microbial shifts were linked to altered salivary cortisol, a key hormone in stress regulation.
The Oral–Brain Axis: A New Frontier
This study builds on emerging science around the oral–brain axis, the idea that microbes in our mouths may influence our mood, cognition, and overall mental health, just like those in our gut.
Much like the gut–brain connection, which has become a major focus in psychological and nutritional research, scientists are now discovering that oral bacteria may interact with the immune and nervous systems in ways that affect emotional balance and stress resilience.2,3
Why Oral Hygiene (and What You Use) Matters
This new evidence reinforces something we’ve long believed: oral health is whole-body health. And it starts with supporting a healthy, balanced oral microbiome, not wiping it out.
Our mouths host hundreds of bacterial species, many of which play a beneficial role in defending against pathogens, supporting the immune system, and even influencing the balance of our gut microbiota. Disrupting this balance, through stress, diet, antibiotics, or harsh oral care products may contribute not only to gum disease and decay, but also to systemic issues including mood dysregulation.4
What This Means Going Forward
This study doesn’t mean that depression is contagious in a traditional sense, or that kissing is a health risk. Instead, it opens up an exciting new understanding of how deeply connected we are, biologically, as well as emotionally.
It also points to new directions in healthcare: in the future, microbiome-friendly oral care could become part of a holistic approach to mental health, particularly for couples or cohabiting individuals.
A Fresh Take on Daily Oral Care
This research is a timely reminder that oral hygiene isn't just about fresh breath and clean teeth. It’s about protecting an ecosystem, one that could influence far more than we’ve previously understood.
So, as science continues to explore the connections between our mouths, our microbiomes, and our minds, daily routines like brushing, rinsing, and choosing the right toothpaste take on a whole new importance.
Because the next time you kiss your partner, you won’t just be sharing love, you might be sharing microbiomes. Let’s make sure they’re healthy ones.
