Trust is an important piece of the foundation of a healthy relationship. Although friends are human and make mistakes, be leery of a friend who's broken your trust one too many times or who does so when it benefits her, writes family systems therapist Ann Smith in "7 Signs That You're in a Toxic Friendship" in Psychology Today. This person will take credit for one of your ideas to advance herself professionally, will make a move on the guy she knows you've been eyeing, and will tell a private story you told her in confidence just because it's juicy gossip. The fact that she's hurting both you and the friendship will be of little concern to her.

Think about your history of making plans together. If you can remember numerous times that he cancelled on you because he had a date or because he wanted to do something with his other friends and didn't invite you along, you're being used. This person has a certain order of priorities, and you're not top on the list, warns psychologist Joseph M. Carver on askthepsych.com. And it should go without saying that a friend who treats you as a filler friend for when there's nothing better to do isn't a real friend at all.

She makes plans with your friends and you're not included. When you really like someone, it's only natural to want to introduce that person to your other friends, in the hopes that everyone will get along. However, if you find that you were only good for the introduction and now appear to be on the outside of the friendship circle, you have cause for concern, says author and gender expert Susan Shapiro Barash in an interview with Canadian Living. This shouldn't be confused with your friends genuinely liking each other and forming independent relationships, she adds. A toxic friend will actively try to exclude you. For instance, she'll invite all of your friends to a party without you.

Friendships are supposed to be reciprocal, give-and-take relationships. In an unhealthy friendship, you're the one who's doing all the giving, writes relationship expert Jane Bluestein on her website. Worse still, when you don't give him what he wants, he acts like he doesn't have any use for you. Toxic friendships require one person to be what the other person wants them to be in order to be accepted. Consider whether or not your friend would still want to spend time with you if you said "no" to his requests.