It is difficult to say.... There is a study done by a doctor who measured blood levels in people taking various doses of oral B12.

This is the table from his paper:
http://www.ncbi.nlm.nih.gov/pmc/arti...able/T1/#TF1-4

Notice the ceiling effect per dose. He stopped at 1mg oral, and notice there is not much difference from 500mcg. I suspect there is a similar ceiling continuing if he had gone to 5mg.

I assume you are taking methyl? Some people, rarely have problems with methylation in the body. This is a genetic thing and not predictable.

You could try using adenosyl form of B12... or hydroxocobalamin.
iherb has this:
http://www.iherb.com/Nutricology-B12...8&sr=null&ic=1

And you can search Google for other brands if you like.

Sometimes using another active form of B12 might help. The body converts them back and forth as needed. Bodybuilders favor adenosyl form as it is supposed to help with the muscles.

Hydroxocobalamin is another active form, less common in supplements and a bit more expensive than the other two...
http://www.iherb.com/Advanced-Orthom...7&sr=null&ic=1

I would try one of these two and see if your side effects lessen or stop.