I can’t respond to efficacy of injectable over ingestible but have given a ton of B12 injections to cattle and cats over the years. Knock on wood, so far I’ve never had one complain - and they will definitely complain about some drugs!
So … if you are injecting in the neck and either you are the horse are squeamish about it, feed a treat, pop round to the heavily muscled hamstring area and do a quick drive by. Literally one quick but smooth motion to inject - practically pushing the plunger as you are inserting the needle followed by a quick but smooth withdrawal. “Oops, sorry about the bug bite, no idea where that fly came from in the middle of winter!” Return to pony’s head and insert another candy in the treat slot. So much easier and, I have found, less likely to cause a reaction in the horse/cow compared to neck injections.
If you are nervous about this technique practice ahead with a ball point pen to judge reaction.
I completely changed my needling technique years ago after doing rounds of a horse with Pen G and figuring out that I got the lowest flinch when it was hamstring day. (Obviously do NOT use the drive by technique for penicillin!) That morphed into routinely doing hamstring injections in cattle because I could move through a herd doling out bug bites (of vaccines or vitamins, not all drugs can/should be administered that quickly) without them thinking much of it.
Fast forward to my boss getting broken from doing routine repro injections on a difficult cow, "How. Do. You. Not. Get. Killed. By. Her?!
" Hauled my ass in at the next scheduled injection and showed her the drive by method. “Dont you want her tied up short?” “Nope. I’m not going anywhere near her head or neck. I value my life, thank you.” Boom, bug bite injection with zero reaction from the notorious beast. Boss still didn’t believe it and thought it a one off. Showed her the next time again. Had her practise with a pen. Boom, a believer was born and I never had to do pm injections on that cow again. (My job was am repro and all vax, etc during my usual hours)
In my opinion, it’s worth a try and you may even find it ends up less stressful than oral meds. For my own horse, if I get the choice of nasty tasting drugs or injectable, I will choose injectable.