Vitamin B12 injections are recommended for very severe deficiencies, such as mine. For me, B12 from dietary sources or tablets is insufficient because the digestibility is low. It is caused by the gastric intrinsic factor (see here: https://proveg.com/plant-based-food-and-lifestyle/vegan-nutrients/b12/). In other words, I would have to eat beef liver and take B12 tablets for months or years to get a good result. In the injection, you will find a much higher dose of vitamin B12, which is also better absorbed. For example, the bioavailability of 1 mg of intramuscular cyanocobalamin is around 10-30%, and the bioavailability of hydroxocobalamin is even 70% (please kill me, but I can’t find the sources right now). So you can get as much as 700 mcg per day from one injection, while oral sources can give you a maximum of 10-20 mcg per day (assuming you are taking B12 all day long at intervals of several hours). As a result, one method will allow you to replenish your B12 levels in a few weeks (assuming two injections a week), and the second method will probably allow you to get a few months or years. I would like to add that “the total amount of vitamin B12 stored in the body is about 2–5 mg in adults” (https://en.wikipedia.org/wiki/Vitamin_B12). So, with a severe deficiency, intramuscular therapy appears to be necessary.
Oral liquid methylcobalamin doesn’t work for me, but that’s probably because I’m too deficient (and, of course, because of the low bioavailability of this form of B12). Besides, I would be wary of methylcobalamin as it is a methyl donor, which may be bad for some of us (look here: https://forum.propeciahelp.com/t/a-warning-about-same-methylcobalamin-and-high-dose -b-vitamins / 32044 and also here: https://ec.bioscientifica.com/view/journals/ec/8/8/EC-19-0199.xml).