Taking methyl form (active) B12 may be more useful for you
than injections.

Here is our B12 thread:
http://neurotalk.psychcentral.com/thread85103.html

One cause of low B12 may be a failure to methylate cobalamin into the active methyl form. This is a genetic failure, and when people show up here with such a low result as yours, it begs the question as to if this is a cause. A DNA test will show if you fail with MTHFR methylation.

A second cause is long term use of acid blocking drugs. This lowers and prevents acid which is needed to facilitate B12 absorption.

A third cause is gluten intolerance or Celiac disease. There are blood tests for this also.

Methylcobalamin is available online, inexpensively, and with levels so low as yours, I'd suggest 5mg daily for 3 months. You chew them up and swallow with some water on an empty stomach. Wait 1/2 to 1 hour before eating. B12 cannot be absorbed well with food present (this is passive absorption to by-pass the errors present in the stomach).

This method works well, and within 3 months you should see higher levels than with injections. The new acceptable lower level in US is now 400.(over 1000 in US units).

Nerve damage from low B12 takes time to correct. It is a slow process.

Getting chronic colds and infections suggest a low Vit D level. That should be tested for and OTC D3 (not the RX form doctors typically give) can bring those levels up. The general rule is 1000IU D3 for every 10 pts to raise. The new lower acceptable level is 50 now.

These two nutrients are commonly low in the US. Try fixing them first, before doing other supplements.
This is our PN forum... there is alot to read there and information gathered by our posters over the last 6 yrs.
So be prepared to do some homework.