If you're not vegan and you eat at least one serving of meat a day, (or if you're taking a multivitamin) you're probably already getting all the B12 your body can use, and supplementation is unlikely to have much effect.
Typically negatives with overuse only exist with the supplements in my 1 minute glance. This is common amongst many different supplements actually. If you eat actual food you get full, so it prevents too much nutrition from being an issue. I don't think B12 is really something your body will reject, we are able to efficiently filter out any extra with minimal load on the kidneys & liver.