Notes from DrKF:

This important methyl donor, with few exceptions, is only found in animal protein. If you are vegetarian and eat eggs, you can still get some vitamin B12. There are small amounts of B12 in nori and shiitake mushrooms (although you’d need to eat 50 grams of dried shiitakes a day to meet your daily requirements); nutritional yeast is also fortified with B12. Spirulina and chlorella appear to actually only contain biologically inactive B12, so don’t rely on them as sources of this vital nutrient. Unless you are dedicated to eating these foods on a daily basis, we generally recommend all our vegan patients, and most of our vegetarian patients, take a B12 supplement. We recommend bioidentical forms (the same types that our body uses) of B12, called methylcobalamin, adenosylcobalamin, and hydroxycobalamin, rather than the synthetic form of B12 called cyanocobalamin. The cyano form is still widely used in supplements, most likely due to its cheap price and stability; I don’t think the cyano is harmful (it’s way below a toxic dose of cyanide), but why go synthetic when you don’t need to? 

Don’t hesitate to get B12 levels tested (and also, if you can, look at a surrogate marker of B12 activity like homocysteine) to make sure you’re getting enough. B12 works with folate. If you’re taking B12, you want to be eating a lot of folate foods or might need a foundational level of folate (around 400 ug/day) in your supplement, too.

Dosage: 500 ug/day.