My MD told me NO caffeine because it is irritating to the stomach, and if you develop an ulcer or an erotion around the band it would have to be removed.

Also, caffeine is an appetite stimulant.
As far as beer goes, my MD said it was a double NO NO! It's carbonated (risk of stetch your new small stomach) and it's full of calories. I was told never to drink anything with more than 10 calories per serving.
That advice is for optimal weight loss of course. Nobody is perfect!