When someone says they have something on the brain, it means that they cannot stop thinking about a particular topic or issue. It can be difficult to find the right words to convey this feeling accurately, but there are some synonyms that can help. For example, someone might say they are fixated on something, or that it is consuming their thoughts. Alternatively, they might describe themselves as being preoccupied with a particular topic, or simply say they cannot get it out of their head. Whether you are trying to describe your own preoccupations or understand someone else's, these synonyms can help you communicate effectively.