It happens all the time: Calling that ex who won't stop watching your Snapchat stories "psycho," saying the friend who's paranoid you're blowing them off is "crazy," telling your moody sister she's acting "so bipolar."

We all slip up and say things that we don't mean. But for the one in four adults and 20% of youth age 13 to 18 who experience mental illness each year, these words can hurt.

"These labels can be very upsetting for people with mental illness because they are often used as shorthand for hurtful processes, mostly having to do with delineating who is one of us and who is 'other,'" Nathaniel G. Wade, a psychology professor at Iowa State University, told MTV News.

They carry "a label and meaning that is related to being not-normal: separate, ill, different, not-like-us."

Wade said that hearing these kinds of phrases used as insults sends a pretty clear message to those with mental illnesses -- which are often invisible from friends and acquaintances -- and can make them less likely to be open about their experiences or seek help.

"They can understand the message 'I am one of these people who are not accepted, included, even loved,'" Wade said. "This leads to further hiding of the [mental illness.] One of the worst parts of this process is that people receive a strong and consistent message that they are not okay. Over time this can lead to pretty comprehensive self-hate, identity problems and despair."

As for why these words seeped into our vocabularies in the first place, Wade said that part of the problem is the need to have "power or control" over one another.

"When people do things that are annoying or even just disconcerting to us, we want to find ways that control that behavior," he said. Because using the labels associated with illness "separate[s] someone out as different and abnormal," the person doing the insulting "gains some leverage."

Even if people argue that these words are "just a joke" or "how they talk," two commonly employed excuses, Wade said that ignorance of the true meanings of the words (and their potential to be hurtful) can have lasting consequences.

"This is the processes that keep people from seeking psychological help, whether they have a diagnosable mental illness or not. This is a real problem. To avoid the stigma, people will suffer with ... problems that we have effective treatments for," Wade said. "So many people I see have struggled with their concerns for years before finally seeking help. So much unnecessary suffering."

For people who don't have mental illnesses, Wade suggests trying to be more sensitive to those who do. This means choosing language carefully.

"It might allow more acceptance and self-acceptance," he said. "This would reduce the additional burden that stigma places on people with mental illness who are already struggling with greater burdens than those without mental illness."