General Synonyms:
* Worry
* Anxiety
* Apprehension
* Unease
* Disquiet
* Trouble
* Care
* Interest
* Consideration
* Attention
More Formal Synonyms:
* Solicitude
* Preoccupation
* Misgiving
Informal Synonyms:
* Bug
* Bother
* Hassle
* Fear
Synonyms related to specific situations:
* For something bad happening: Dread, apprehension
* For someone's well-being: Care, solicitude, regard
* For a specific issue: Issue, matter, topic
The best synonym to use will depend on the specific situation and the tone you want to convey.