There is a rare condition, pernicious anemia, where the patient is unable to absorb oral B12 because a factor produced by cells that line the stomach don't make that factor or the lining of the stomach is absent from surgery or other rare conditions.
If you cannot absorb oral B12 then injection is the only option other than massive oral supplementation to try to get sufficient amounts into the bloodstream. There are very rare cases other than absent intrinsic factor that can impair B12 transport.
There are two reasonable approaches depending on whether you are treating a symptomatic person... they need B12 now so give the shot to correct the deficiency while you do the evaluation. But if the patient has no symptoms it is perfectly reasonable to try oral supplementation and follow up with a blood test to see if the B12 deficiency is corrected.
Those doctors who give you a B12 shot to boost your energy are quacks or crooks, IMO. YMMV
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Men plug the dikes of their most needed beliefs with whatever mud they can find. - Clifford Geertz
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