Does alcohol cause any vitamin deficiences? If so how do I know if I need to take extra supplements?
Yes. Alcoholism is associated with, among other things, B12 and Folic Acid deficiencies. You should not, however, have a problem if you eat right, exercise, and drink alcohol only in moderation. If you are an alcoholic then vitamin deficiency is only a part of the problem, and the problems won't improve until the alcohol abuse stops no matter how many supplements you take.
Yes. Alcohol depletes the body of many vitamins, especially thiamine (vitamin b1), folic acid, and pyridoxine (b6), and may also reduce vitamins c and d. People in recovery are generally recommended to take thiamine 100 mg, Folic Acid 1-2 mg and a general supplement (one a day, centrum etc.). The most important thing is to stop drinking, if you are drinking to excess. See your doctor or a counselor.