I agree with kenkeith but will also say that 8oz. of light beer is an extremely minimal dose of alcohol, especially if you are only doing it occasionally. I am on coumadin and drink more alcohol than that without any problems. But talk to you doctor, as kenkeith says, because everyone is different.
It is known that alcohol can cause coumadin to be less effective by increasing the risk of blood clots. Moderate intake of one drink a day is usually OK for most people. You should talk to your healthcare provider about an alcohol intake that is safe and acceptable to you.