Is it allowed in Union Europe to bring with you (i.e. in hand luggage) xanax (or valium or any similar anxiety medication)? for personal usage, most likely one box.
There are two issues that sometimes get confused:
What's allowed on the plane - This is determined by security rules. If a prescription drug is not a liquid that takes you over the liquids limit, then it's allowed. If it takes you over the limit, it's generally still allowed as long as you can prove that it was prescribed to you. Prescription drugs in pill form are not a problem at all.
What's allowed into the country / EU - This is determined by customs rules of each individual country. I don't know the exact EU rules, but I would guess it's similar to US/Canada rules, where less than 1 month supply of a prescription drug is certainly ok, but for some stronger/psychoactive drugs you need to have proof of prescription.
In any case, DO NOT try and hide the fact that it's Xanax by putting it into another bottle, etc. Anything like this will just be a red flag for any official, either security or customs. Prescription drugs should be in original packaging, ideally with your name and prescribing doctor's name / contact info on the packaging.