General synonyms:
* Teaching
* Belief
* Principle
* Theory
* Creed
* Tenet
* Precept
* Dogma (implies a rigid or inflexible set of beliefs)
* Philosophy (if it's a broader system of beliefs)
* Code (if it's a set of rules or regulations)
More specific synonyms:
* Theology (for religious doctrines)
* Jurisprudence (for legal doctrines)
* Policy (for doctrines that guide action)
* Ideology (for doctrines that shape political thought)
* Canon (for a body of doctrines considered authoritative)
The best synonym will depend on the specific context in which you are using the word "doctrine." Consider what you want to emphasize:
* Formal instruction? Use "teaching" or "precept."
* A set of beliefs? Use "creed," "tenet," or "dogma."
* A guiding principle? Use "principle," "theory," or "philosophy."
Think about the nuance you want to convey, and choose the synonym that best fits your intended meaning.