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Kissing is a fundamental part of most romantic relationships. Yet it might be a bit riskier than most would assume, particularly if your partner struggles with their mental health. A recent study of newlywed couples found that swapping spit can have unexpected side effects.

According to a new study, you can catch someone’s depression and anxiety by kissing them.

Researchers from Iran, India, Italy, and the U.K. screened 1,740 couples who had been married for six months and were living together. Of those couples, they recruited 268 for the study, all of whom had one spouse with insomnia with anxiety and depression, and one without. They found that depression and anxiety can be passed from one spouse to the other through the oral microbiome, which is transferred when kissing.

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The oral microbiome refers to microorganisms found in the human mouth. Oral microbiota dysbiosis is an imbalance in these microorganisms, which the researchers explained “is significantly cor with various neuropsychiatric disorders, including autism spectrum disorder, dementia, Parkinson’s disease, schizophrenia, anxiety, epilepsy, and depression.”

The researchers took the oral microbiome composition and salivary cortisol levels of participants on the first day of the study, and again after 180 days. The spouses who previously did not display attributes of insomnia with depression and anxiety did so by the end of the study, and their oral microbiome makeup changed, too.

“The researchers found that after six months of marriage to a partner with insomnia with features of depression and anxiety, healthy spouses showed increased depression and anxiety scores, and poorer sleep quality,” Paul McClure reported for New Atlas. “The researchers observed that the composition of the bacteria found in the mouths of healthy spouses changed considerably, becoming increasingly similar to their affected partners.”

The changes were most pronounced in females, so ladies, be careful of who you are kissing!

: How Often You Kiss Reveals How Long Your Relationship Will Last

Kissing can have other effects on your health, too.

Research shows that during a 10-second kiss, as many as 80 million bacteria are transferred. This, of course, means that germs and viruses can be passed between partners, even more easily than depression and anxiety.

“Kissing can open us up to a host of negative bacteria and viruses,” Ariane Resnick wrote for VeryWell Mind. “You can transmit germs for everything from cold sores to a cold to COVID. Illnesses such as Mononucleosis, known as the ‘kissing disease,’ can leave you incapacitated for months.”

Surprisingly, though, the spread of bacteria during kissing isn’t all bad. One study found that kissing the same person over a period of time can actually boost both people’s immune systems. Good bacteria are also transferred when kissing, some of which help to fight off infections.

: Science Says 15 Minutes Of Cuddling Can Transform Your Relationship

Kissing also triggers the release of positive hormones.

“During a kiss, lip sensitivity causes our brain to create a chemical cocktail that can give us a natural high,” science communicator Emer Maguire explained. “This cocktail is made up of three chemicals, all designed to make us feel good and crave more: dopamine, oxytocin, and serotonin.”

“Like any cocktail, this one has an array of side effects,” she continued. “The combination of these three chemicals works by lighting up the ‘pleasure centres’ in our brain.”

Research shows that the increase of oxytocin, otherwise known as the “love hormone,” when kissing seems to be specific to men. This hormone helps them bond with their partner and boosts affection and attachment. Dopamine and serotonin are “feel-good chemicals” that boost your mood, making you feel happy, motivated, and focused. Kissing can also lower cortisol levels, which correlate with stress.

So, while you could catch your partner’s depression and anxiety through kissing, the side effects of locking lips are not all bad. Still, kiss at your own risk.

: 7 Scientifically-Proven Benefits Of Showing Affection To Your Partner

Audrey Jaber is a writer and associate editor with a bachelor’s degree in journalism.