Physicians prescribe 10 mg as the initial dose. You should NOT use the 20 mg dose unless the 10 mg dose fails you. Unfortunately the tablets aren't scored, so you can't buy a 20 mg dose and cut it in half. That would be great financially, but pharmaceutical companies have caught onto that.
5 mg is the dose that is used for daily use. Do not take the 10 or 20 mg dose within 36 hours of each other.
It is always a bad idea to drink alcohol when using Cialis, Viagra, or Levitra. Sure, one drink probably won't hurt, but if you drink excessively it will cause your blood pressure to drop. Besides, who wants to be so drunk that he doesn't remember a night of great sex?