Where I live, they won't let you donate if you've blown coke or any other powder in the last year, booted up at all, or used rolls or any other so called "designer" drugs. I don't recall if they asked about weed or not. I just made it a point not to burn for a few weeks before I donated. They'll ask you a list of questions of what you may or may not have used. Then, if you answer no to everything, they'll take the blood. They'll give you a paper with an assigned anonymous code number that, if you felt uncomfortable being straight forward at the interview, you can call the number on the paper and tell the person on the line not to use the particular code number that was assigned to your pint of blood. That way it's all anonymous. It may different were you are.