From the Manufacturer:
Vitamin B-12 deficiency can occur for several reasons. First, as we age the intestines do a less effective job of absorbing the nutrient from the food we eat. Secondly, the primary food sources are meat, eggs, and diary. Vegans and vegetarians are at greater risk of developing a deficiency because of this. Thirdly, individuals who have had gastric bypass surgery or other procedures involving the digestive tract may be see levels decline. The first reason is the most common.