of course, another possibility is blood type.
If you have type 'O', or type 'A' and that is plentiful in stock, they may decide that taking your blood wouldn't be necessary, with your situation in mind.
If you happen to have a rare blood (i.e. type AB), they may take it anyway.
Like everyone has said, it may depend on the blood bank doing the collecting.
However, I had been told that with a lot of antibiotics I was on, it wouldn't be accepted--this was a long time ago, though.