What does "go to someone's head" mean?

Native speaker’s answer
Rebecca
To "go to one's head" is an informal expression which means to become conceited. You might use this expression to joke about how a friend will become conceited after being successful at something. Ex: I know you just won a gold medal, but don't let it get to your head. Ex: She won the contest but soon started acting conceited because she let it get to her head.