Formal:
* Obsessive: This word highlights the repetitive and intrusive nature of the thoughts or behaviors.
* Fanatical: This word suggests extreme enthusiasm or devotion, often bordering on unreasonableness.
* Zealous: This describes a passionate and enthusiastic commitment to a cause, but it doesn't necessarily imply negativity.
* Fixated: This emphasizes the intense focus on a particular object, person, or idea.
* Compulsive: This describes an irresistible urge to perform certain actions, even if they are harmful or irrational.
* Maniacal: This implies a state of extreme excitement or frenzy.
Informal:
* Obsessed: This is a more colloquial way of saying "obsessive."
* Freak: This term can be derogatory and refers to someone who is considered eccentric or unusual.
* Nutcase: This is a slang term for someone who is considered crazy or irrational.
* Hardcore: This term suggests extreme dedication and commitment, often used in relation to hobbies or interests.
* Die-hard: This term is similar to "hardcore," but emphasizes unwavering loyalty.
The best word or phrase to use will depend on the specific context and the desired level of formality.
It's important to remember that obsession and fanaticism can be serious issues, potentially leading to negative consequences. If you are concerned about someone's behavior, it's best to seek professional help.