emmie, Its very rare for the person's tolerance for alcohol to make a difference to their blood alcohol levels. Medicines with alcohol in that are necessary to a person's health, are also rare and it would be amazing if they had to take enough of the med to make it register, even on a blood test.
No one is saying that you can't have a drink in you own time, just that if you do so, it must be out of your system before attending work. I don't think that is unreasonable.