The only things they need to know are whether it impacts on your work, and whether it compromises your, or your colleagues health and safety.
So if you're operating machinery/around vulnerable children or adults I imagine medication may be important to share. Sitting at a desk....not so much.
This is US law, but I think ours is similar -
http://www.ehow.co.uk/info_8421794_d...-employer.html
And I thought this would be useful -
http://www.mentalhealthworks.ca/empl...ies/disclosure