I hope Rose chimes in, but her site is informative enough.
If you take B-12 as a part of a B complex or multi-vit- it is CYANOcobalamin.
METHYLcobalamin is the oral sub-lingual type.
If you are taking a B-12 supplement alone, your B-12 test results will be skewed. Read Rose's suggestions on what to do prior to blood work tests.
My levels were (at first) in the 450 range and deemed 'normal' by my idiot PCP at the time.
After taking methyl- the labs flagged me, and my present PCP had to make notation that I take supplement for B12.
My range now is in the 1200-1400 levels.
Be wary of any doc that says 'normal' (even 'abnormal') - get the results and know your numbers. Then compare them and use the Liza Jane charts
to track differences over time, and succeeding testing.
Bob B