When people are severely low in B12, and you flood the system with it with a shot, the body responds by making new red blood cells if you are severely anemic. It is important to realize that 1000mcg entering your body suddenly is a huge amount...when 2-4 micrograms is the RDA it is used to using.

Oral is not absorbed totally, so it is much more gentle.

What happens then in severely anemic people who are low in B12 is that the red cell factory goes into overdrive, and uses up the potassium you have hanging around. This is a survival mechanism so the potassium may become low temporarily. Low potassium gives weakness and severe fatigue, and if really low will result in cramping of muscles. Eating foods high in potassium may help this. Cantaloupe, V8 juice, bananas, oranges, potatoes, meat, are good sources. V8 has more potassium in a 12 oz can than many RX potassium capsules!

This may be the reason for the "fatigue" after the shots.

I really think using the oral route, is more efficient and gentler for you. 2000mcg is okay, but if you know your number is really low 5000mcg daily for a month or two would be better. Try the 2000mcg now, once a day, on an empty stomach, and you should see some response. Some people claim within a week, but I don't see how that can be really. Expect to see some effects in a month or so.