I wouldn't worry about having an unfair advantage - just be sure the medication won't interfere with your comprehension during the exam. It would really suck to be all chilled out for the exam, only to find out you don't understand any of the questions. That wouldn't fall into the "advantage" category. Go ahead and smack me if I've added to your anxiety.

What about yoga or meditation? Maybe not for the exam in May, but for anxiety issues in the future? There are no harmful side effects, and it's non-narcotic.