When people think about bullying, they don't usually think about someone's friend being the one who might be picking on them.
Sometimes someone might claim to be your friend, but they actually show bullying behaviour.
If this is the case, some people might describe them as a 'frenemy' - that's a combination of the words friend and enemy.
An anti-bullying expert says that frenemies can often be a big problem in schools.
Val, from Bullying Intervention Group, runs workshops to help solve friendship issues. She said that "in friendship groups you can have arguments" but sometimes "there can be really nasty behaviour from friends to other friends".
She told Newsround a toxic friendship is "when someone is being deliberately left out of the group" or if the friend "tries to get them involved in something they wouldn't normally do".
Val says frenemies can especially be a problem online, when group chats carry on after school time and arguments can "spiral out of control".
The anti-bullying charity Kidscape says there are certain signs which might suggest a friend isn't a good friend:
- They might say "brutally honest" things to you which are unkind or hurtful
- Put pressure on you to do things you don't want to do
- Laugh at you, or encourage others to laugh at you
- Talk about you behind your back
- Deliberately leave you out of group chats and activities
- Share things about you online
- Make you feel bad about yourself
Val says social media can make things worse: "Don't get involved in other people's arguments, tell an adult if someone is being bullied online. Treat your friends online, the same way you would face to face. It's important to stay safe online but also to stay nice online."
As well as recognising a bad friendship, some of you have been giving us your thoughts on what makes a good friend. For example, someone who is there when you need them and very trustworthy as well as someone who lets you join in with games.
What do you think makes a good friend, let us know in the comments below.
And if you are worried that you or anyone that you know is being bullied, speak to an adult that you trust about it. That might be a teacher, someone in your family or another adult that you trust, like a family friend.
BBC Own It has lots of advice too:
You can find more help and advice on the new Own It app.
You can also call ChildLine for free on 0800 1111.
There is also a special area on the Childline website with lots of help and advice.