Low serum Vitamin B12 levels may be associated with many medical conditions related to an inadequate vitamin B₁₂ intake or reduced gastric secretion of intrinsic factor, which inhibits absorption. As a dietitian in the clinic, I typically ask my patients about their overall dietary patterns to see what sources of Vitamin B12 they may be lacking and we then work towards adding some of these foods and/or supplementing with a Vitamin B-12 supplement. According to the 2005-2006 National Health and Nutrition Education Survey (NHANES), the average vitamin B₁₂ intake from food for adults 20 and over was 5.64 mcg. Vitamin B₁₂ is found only in foods of animal origin or in fermented foods where bacteria produce the vitamin. Organ meats are the best sources of vitamin B₁₂ (liver, kidney, heart, and pancreas), followed by clams, oysters, extra-lean beef, seafood, eggs, milk and yogurt, chicken, cheese, and miso (a fermented soybean product). For people who lack intrinsic factor, it has been found that a 1,000 mcg daily oral dose can substitute adequately. Since the primary sources of vitamin B₁₂ in the diet are animal products, vegetarians have a high risk of developing B₁₂ deficiency. Therefore, supplements are recommended for vegetarians to prevent deficiency. Please check with your doctor or your medical team before beginning any type of supplementation.  

While I am not aware of any chemotherapy treatment that may cause B12 deficiency 14 years out, if you are B12 deficient it should be treated through diet or supplementation.