When I took b-12 as a kid
I kept having this sharp pain in my ass every time I injected it so I stopped. It doesnt seem to bother me anymore though. Only use the Synthetek one, it is the best one out there.
The reason they say to inject B-12 is because the molecule is so large that it hard to absorb a lot of it, so injecting it is a much more efficient way of absorbing it.