why do you want to take oral b12 after you have had an injection?
Usually patients that take B12 injections can not absorb it-- this is especially true in elderly people who have changes in their GI tract that limits ability to absorb this vitamin.
If you have been taking B12 orally and are still deficient, you could try B12 sublingual (under your tongue); if that doesnt work then consider B12 injections.
Although no clear toxicity from massive doses of B12 have been recorded (as of 1998 Food and Nutrition Board- my reference book), neither has any benefit been noted with excessive intake with patients with adequate vitamin B12 status.
B12 Sources: Meat/poultry, fish, oysters, clams, eggs (yolks esp); milk/cheese and yogurt (although they contain less than the meat sources). Vegans or those not taking these foods mentioned may need B12 supplements
Check with your physician or health professional.
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I've been doing a lot of research on B12 injections the last week or so (I have Crohn's and have had a colon resection) and in one of the articles, I read that you shouldn't take injectable AND oral. Sorry, can't hyperlink which article it was or would let you know.