It starts with a missed call. Maybe she doesn’t text you to tell you she’s running late. Maybe he makes a slight remark about the clothes you’re wearing. You want to hate the person so badly, but you’ve been friends for years. You can’t turn your back now.

Your health would prefer you did, though. A growing body of evidence in psychology research is finding that a unique mix of emotional closeness and personal conflict makes frenemies some of the unhealthiest people to be around. Not only do they have the power to drill straight to that thing that hurts you the most, but they can soften it with a false aura of friendly honesty. The truth is, your frenemies are making you unhealthier.

For starters, the people you love to hate (or vice versa) usually come with a maddening dose of unreliability. Even when they say they’ll be 15 minutes late, they somehow manage to show up even later, filling us with anxiety the next time a new set of plans rolls around. “There’s this uncertainty,” Julianne Holt-Lunstad, of Brigham Young University, told the BBC. “Are they going to come through for me or bring up a painful topic again?”

Frenemies don’t just let us down. They fill our physical beings with a constant sense of worry, which triggers a cascade of hormone releases and raises our blood pressure immensely. Holt-Lunstad and her colleagues published a study in 2003 that found just being in the same room as a frenemy — or, in the jargon of psychology, an “ambivalent relationship” — resulted in a spike in people’s real-time blood pressures. It was the anticipation that ate away at them, the thought of having to engage in stilted, often passive-aggressive conversation.

Proper friendships don’t come with this baggage. At least, they shouldn’t. A wealth of data confirms that positive social interaction raises the levels of oxytocin (the “love hormone”) and serotonin (the “happiness hormone”) in people’s bodies. Meaningful time spent with loved ones — or even liked ones — also helps to lower biomarkers for negative physical reactions, such as heart rate and cortisol levels, which indicate stress. In Holt-Lunstad’s study, people’s blood pressure fell when they interacted with supportive friends.

We feel so much happier around genuine friends because we trust them, not just to pick up our laundry or watch our dog while we’re away, but to avoid sensitive topics in conversation and know when to flip the switch between joking and honesty. Frenemies may come through in the clutch once in a while, but what they gain in small amounts of trustworthiness, they lose in long-term letdowns. And unlike our responses to straight-up enemies, frenemy conflicts tend to sit with us longer and keep us quiet, for fear of rocking the boat.

“They are almost part of our self,” said University of Utah psychologist Bert Uchino to the BBC, referring to a frenemy’s closeness. “So what they do and say can have a much bigger effect on us. And because emotionally you have a stronger connection, they have a stronger opportunity to hurt.”

As difficult as it may be, the best strategy for overcoming frenemy hurdles is through communication. You won’t be able to turn the relationship into an actual friendship all the time, but each cutting remark or missed call could actually be an act of negligence, not malice. Sometimes people are absent-minded. They forget to consider how their actions affect others. And while it’s easy to swallow one dropped set of plans after another, it’s important to consider how their actions affect you and your health.