As ATCOs (and I assume pilots) we have a responsibility to ourselves to know that we are or are not fit for duty, this is either via your own GP or self assessment.
If you feel like crap or suspect that you're not going to be able to make the full shift then you don't go in.
There are CAA guidelines as to which medication you can take and work and those which you can't.
You either then self certify or if you really need a sick note, you get one or you don't and you go to work.