You would make me look this up, wouldn't you? hehe I don't know B12 info off the top of my head.
2.4mcg per day is the normal dose for adults. However, as people age, they may lose the ability to absorb it. That's why the doses are very large. The 100mcg ones are for seniors.
Now, let's go over a few things that I do know off hand
Are you a vegan or a vegetarian? If you're a vegetarian, I don't know how the heck this happened. If you're a vegan, it may have happened for a few reasons:
1. You may not be using an active B12 source in your daily diet. Active sources include most fortified soymilks, Yves products (I know their's is active, not sure about others) and nutritional yeast.
2. You may not absorb it from vitamins as well as other people. I know I do not, but it is supposed to be an uncommon problem. I also have the same problem with iron.
3. If you are a lacto-ovo, have you been dieting and leaving out dairy and eggs directly but not replacing the B12 in your diet? For example, you'll have products with whey in it, but you won't eat a piece of full fat cheese.
Allow me to say this - it is kinda unfair of us to criticize your brother's GF's choice. There are people, albeit a small number, that find it ultra hard to get B12 and iron from vitamins, so foods offer a better choice. So, before people jump up and down at her, let's concentrate on your condition, not her's.
Does this help you?