I read some questions in this site regarding B12. Some said 50 mcg was required per day, while some said 1000 mcg.
While Vegan Society says 3 mcg per day atleast.
What is the amount of B12 required per day to have a healthy body?
For vitamin B12 oral intake, 1,000 mcg daily is both safe and sufficient.
Update: A recent randomized controlled trial of vegetarians and vegans who were marginally B12 deficient showed that 50 mcg B12 taken daily was sufficient to correct B12 status. Effect of two different sublingual dosages of vitamin B12 on cobalamin nutritional status in vegans and vegetarians with a marginal deficiency: A randomized controlled trial
Guidelines from NIH suggest that we need 0.4 - 2.8 mcg B12 depending on our stage of life. The large difference between requirement and recommended supplemental intake arises from the different ways that vitamin B12 is absorbed in the body.
No harmful effects have ever been shown from consuming too much vitamin B12, and no maximum dose has been identified. Vitamin B12 is water-soluble and excess vitamin will be rapidly excreted in the urine over the course of a few days.
However, take care with multivitamins and b-complex vitamins because vitamin B6 has a tolerable upper limit of 100 mg/day in adults, and excess B6 can be harmful.
See also: Can too much B12 be harmful?
Vitamin B12 in chicken eggs is poorly absorbed compared to other animal sources, so ovo-vegetarians are also advised to take B12 supplements.
Oral doses of vitamin B12 of at least 1000 mcg are considered just as effective as intramuscular injection.
A 2011 study of 5600 Canadians showed that about 4.6% of the population had low vitamin B12 status (<148 pmol/L in the blood).
Among Canadians, typical B12 concentration in the blood is 350 pmol/L when not taking supplements. On supplements, B12 concentration may rise to 550 pmol/L or higher.
I am a vegetarian for about 6 years and my experience has been that if i do get my blood checked once a year, for B12, I then decide if and how much B12 to supplement. There are many different body types and everybody works differently. But when you stay checked you do get a pretty good feeling about your own body, which is the most important and where you want to be, healthy and strong.
You need far less daily than most supplement regimes provide. However, this is deliberate: there is an issue with bioavailability and absorption. Still in most cases, medical professionals will advise supplementation of no more than standard over-the-counter supplements 25-50µg once per day. Note than this is "micrograms" (a.k.a. mcg) and milligrams (mg) is far in excess of what most people need.
Usually supplementation is only needed for vegans (or vegetarians with low egg and dairy intake). There is no immediate risks of overdose but many health food companies will market products with far higher doses than necessary. Unless you absolutely need these, it is not worth the additional expense which adds up over the course of a daily medication. If you have particular deficiency concerns, you may need injection or other treatment to ensure absorption. Vitamin B12 can be accumulated and stored in the body. If you are experiencing extreme deficiency, presenting symptoms, or at risk of chronic illness, you may be put on a course of higher dose supplementation to restore these stored vitamin reserves.
This is a serious health concern so if you have any concerns about B12, consult a medical professional. Any GP can order blood tests and monitor your vitamin levels. There are many ways to manage vitamin deficiency to varying degrees of severity and how your specific case develops over time so this matter is best discussed with a qualified doctor.