Another thing to understand is that your initial test is very very LOW.... lab ranges in US are not accurate anymore and report levels in the 200's and "normal". Doctors then follow THAT without getting continuing education on the subject.

The thread linked by Dr. Smith is my thread, and explains this.

You should test above 500 to show benefit. You would have to wait a week after an injection to get a useful reading, as the injection lasts about 72 hours in the blood. Waiting a week may show more accurately. Doctors ASSUME every one should get monthly shots. But none of us are identical in the way be process B12.

B12 has to enter the spinal cord to get into the CNS. When blood levels are low, your cerebrospinal fluid is low, and this is very bad. MS and Fibro people have been tested in studies and typically show low B12 in the spinal fluid. The B12 in the CSF mirrors the blood and if your blood is very low, below 500, your CSF will be low too.

Pennies a day of a quality product that has no toxic upper limit?
There are not even drugs, this safe and inexpensive. You can take care of yourself in this regard, and only use the doctor for testing. Don't expect a clear interpretation by most doctors in fact. On my thread is a link the new medical information from American Association of Family Physicians. Please read that and you will understand this topic much better than most doctors today.