General synonyms:
* Prudent
* Wary
* Circumspect
* Discreet
* Heedful
* Mindful
* Careful
* Attentive
* Thoughtful
More specific synonyms:
* Apprehensive: Worried or fearful about the future.
* Chary: Hesitant or unwilling to do something.
* Timid: Lacking courage or confidence.
* Leery: Suspicious or distrustful.
* Guarded: Carefully controlling what one says or does.
* Conservative: Avoiding risk or change.
* Defensive: Protecting oneself from harm.
The best word to use will depend on the context and the specific meaning you want to convey.