What to do if you know your boyfriend is lying?

Ask him the reason, talk to him and make it clear lying is not gonna help. If he repeats, then you know what to do. Can't trust liars.

All you can do is accept it or reject it. Those are your two choices.

i confronted a friend when i knew she was lying to me. and well that ended the friendship. but i think she was a sociopath.

Wait and watch him hang himself or call him out. If you call him out though, he's most likely been trained to deny and deny again. So then he'll turn the situation onto you, making you look like you're insecure and nuts, thus never having to acknowledge the lie.
In my humble opinion, I'd wait and catch him when he stumbles over himself, he always will, then call him on it.
Unless your man is a decent man, who acknowledges his issues. Then say something like healthy people do.

No trust. No relationship.

Don't except it. Listen to your gut and don't be left broken. Quit first.🙂

Well, do you mean "know" as in "really believe" or "have proof"? Because it sort of depends on which it is. If it's the former, getting solid proof of some sort would probably be the place to start. Otherwise, just stay vigilant, and if the chance to catch him out comes up, take it

I'd probably murder him wby

Say you know.

Ignore it the first time.
Ask about it the second
Make a big deal about it the 3rd
4th? head for the door.

i don't lie. but chicks always lies to me

I went for the confrontation and made her confess

Wash his mouth out with soap😉

Call him out on it

truth serum?! 🤐

Depends on what the lie involved: something trivial or relationship-altering.