They don't say that deficiency is irreversible but the progression of central and/or peripheral neuronal damage caused by deficiency status.

The sooner you correct deficiency status the less central /peripheral neuronal damage will progress.

B12 methylcobalamin can also be bought in a spray form the sublingual or spray forms of methylcobalamin are the best, better in fact (more effective) than shots that some people have from the doctor.
The reason for the spray/sublingual is to allow some of the B12 to be absorbed directly through the tissues in the oral cavity rather than pass through the digestive system where they may be degraded. You may find you have B12, folate, B6 in your mulitvitamin but generally speaking the forms in multi's are the cheaper forms that are least well absorbed and require activation (further processing steps) before your body can use them. It may be the case that your body manages to convert from inactive to active form without problems but for others it's better safe than sorry, the least steps required the less chance of things not working.

Those who are concerned about the prospect of Alzheimer's/dementia and who are striving to keep homo-cysteine levels low may also want to consider activated folate and the activated form of B6

Long term treatment with metformin in patients with type 2 diabetes and risk of vitamin B-12 deficiency

The examples of active forms of B vits I've provided come from IHERB because they offer cheapest shipping to UK (you may find same examples cheaper else I haven't checked shipping to where you live) Code ~~~~~~ saves Iherb Newbies $5.