Low Vitamin B12

The most common cause of B12 deficiency in adults is from pernicious anemia.  Wikipedia has a good description of pernicious anemia. 

I am assuming your low vitmamin B12 level was determined by a blood sample, not another way of measuring B12. 

This type of B12 deficiency is also called megaloblastic anemia.  Was your hemoglobin and MVC checked before you started taking supplementation?  If these parameters were abnormal, then you likely have pernicious anemia.

This type of anemia is often found in elderly people or in people whose stomach cells (parietal cells) start atrophying (disintegrating) because of a person's own body attacking his/her stomach lining.

Oral supplementation is fine.  Overdose is not an issue.  Another way of getting this is by having monthly B12 shots.

If you have pernicious anemia, you must take B12 supplementation life long in either of the ways mentioned above.

If you do not have pernicious anemia, then there are other questions I would have, that would be best addressed by seeing a naturopathic doctor. 

Example:  Do you have chronic heartburn?  What is your digestion like?  What is your health like (detailed health history taken by a healthcare professional)?  Do you have any pre-exisiting illnesses?  Are you under tremendous stress?  What is your nervous system like?  etc.,etc.,etc.

If you do not have pernicious anemia, and you do not have a pre-existing disease (example, neurological disease, diabetes, etc.), then it is unlikely that you would need to take B12 lifelong if the underlying condition was addressed.

Non-pernicious anemia B12 deficiency can also be cause by food allergies, as determined by an ELISA IgE/IgG4 food allergy test.  The inflammation form a food allergen can impair the absorption of vitamin B12.