People searching for the "fastest way" to fix B12 deficiency are usually hoping for a product recommendation. The accurate answer is that the right approach depends on why you are deficient, and establishing that is a clinical task.
This article describes what published sources document. It is not a treatment plan, and you should not act on it without a clinician — partly for the reason in the final section.
Why the cause changes the approach
NIH ODS explains that parenteral administration — usually intramuscular injection — "is typically used to treat vitamin B12 deficiency caused by pernicious anemia as well as other conditions... that result in vitamin B12 malabsorption and severe vitamin B12 deficiency."
The logic is direct: if the problem is that you cannot absorb B12 through the gut, an oral supplement is working against the same obstacle that caused the deficiency.
Where oral routes are documented
NIH ODS notes that people who have difficulty absorbing B12 from food generally absorb free B12 normally, so their deficiency tends to be less severe. It also cites evidence that high-dose oral cyanocobalamin "might be equally effective" as injections in Crohn's disease, and that high-dose oral methylcobalamin appears as effective as hydroxocobalamin injections after gastric bypass surgery.
A prescription nasal gel spray is also described.
The reason self-treatment is a poor idea here
Two reasons, both from the sources. First, NIH ODS emphasizes that neurological symptoms can occur without anemia and that early intervention matters "to avoid irreversible damage" — so the stakes of getting it wrong are not trivial.
Second, and less obvious: supplementing B12 can improve the anemia while leaving the underlying cause unaddressed. If that cause is pernicious anemia or atrophic gastritis, it is a condition that warrants diagnosis in its own right.
What to do instead
Get tested, establish the cause, and let the cause determine the approach. That sequence is what the evidence supports. If you have neurological symptoms, treat it as time-sensitive.
Our free assessment helps you prepare for that conversation.
References
This article summarizes published guidance from the sources linked above. It is general information, not medical advice, and it cannot tell you whether you personally have a deficiency. Only a clinician ordering the right test can do that. If you have symptoms that concern you, speak to a doctor.