Xenophobia - a phobia of anything unfamiliar. Though some anxiety is natural and healthy when encountering anything unfamiliar, xenophobia is an exaggerated form of this anxiety.
Xerostomia - excessive dryness of the mouth due to insufficient saliva. This symptom can be caused purely by anxiety, or may be a side effect of medication. Also see salivation.
XR - an acronym for "eXtended Release." XR is used to describe medications that have been designed for slow and prolonged effect to address persistent symptoms or pathology that are not very episodic in character.