The injections do wear off quickly for some people.

One of the references I use, states that the cyano form only remains in the blood serum for 72 hours.

I have read that methyl B12 is cleared faster even.

Our bodies are designed to have 2-10mcg absorbed orally when things are working well. (intrinsic factor present and active).

So I tend to recommend oral high dose 5mg on an empty stomach daily, to mimic the normal absorption pattern.

Over the years there have been some who report this swinging effect up and down. I think the B12 is just being sucked up by the liver and brain very fast in people who were very low. The oral way may be more gentle.

This product is partially activated form of B12. It may be better than cyano orally, but less quickly utilized and hence slow down the swinging responses:
http://www.iherb.com/product-reviews...lets/2454/?p=1
I would take it on an empty stomach too.

I'd watch your folic acid intake for a while. Try not to supplement this yet, and see if you tolerate better. If you do take folate, take the methyl form..MetaFolin. People can have genetic errors in activating these two vitamins -- it is called methylation errors.(MTHFR)