With the appropriate care and cleaning you can reuse your bottles and caps many times. You should always check your bottles for internal scratches that can harbor unwanted bacteria, and always make sure the caps' threads are not stripped or damaged in anyway. As long as you take care of your bottles and keep a close eye on their condition, you shouldn't have any problems reusing them.

PLEASE NOTE -- The ring that snaps off of the Mr.Beer® caps the first time they are used is a tamper proofing ring for commercial use, and will not affect the seal of future batches.