This was the answer to a similar question asked elsewhere.
......how alcohol affects the levels of the hormones in birth control pills,......... it actually raises the levels in your bloodstream. This is because alcohol is metabolized in the liver, as are the birth control pill hormones. So, if the liver is busy metabolizing a lot of alcohol, then it can't break down the hormones in the birth control pills as well. This is not likely to be a big problem if you are only having an occasional drink