similarly, i have low b12 and i think i have IBS. my doctor prescribed me oral supplements and they shot my numbers right back up again. i think the main cause is i don't eat a lot of b12 rich foods (and perhaps coupled with my supposed IBS). I second the thought that the shots are a scam, i think it's hooey.
My Dr told me we'd try the oral supplements and if that didn't work then we would move to shots because that meant i wasn't absorbing any B12 in my gut - to me it sounds shady to offer B12 injections to people who are not low in b12 and who do not require supplements otherwise.