No I don't think so, although your employer may have a clause in your terms and conditions of employment which states that you should not use any drugs or medication which impairs your ability,, especially if you are operating machinery, etc. Also, your employer has no right of access to your medical records from your GP without your written permission.,
If you're mailing from Ireland, then no, although the 2005 Safety Health and Welfare act does allow an employer to suspend/sack any employee who is intoxicated - including prescription drugs.
depends on what drugs youve been perscribed and what job you do,wouldnt like to think your on some sort of sky high tabblets at side of me on a motorway,thats for everyoens saftey,just tell your employer,it covers your back