Cyanocobalamin can encounter lots of issues being converted to methylcobalamin and adenosylcobalamin. If you are taking a "regular" B-12 and experiencing bad effects, you should immediately stop taking it. You could be damaging your liver.
I found a huge world of difference when I started avoiding all forms of cyanocobalamin and using jarrows sublingual methylcobalamin instead. Almost euphoric for the first few days. My dad noticed the same thing (it is often genetic).
If you are deficient, you will notice really good effects from the first time taking it, but you will need to add some potassium to your diet until you get the deficiency corrected in your body. Those who are extremely deficient notice headaches and muscle cramps, since the increased metabolism of methylcobalamin will be stripping potassium out of your body if you don't have enough in your diet.
I did notice increased sleep duration the first week of taking it. Methylcobalamin is important in the production of Melatonin, so my guess is that my natural Melatonin production picked up after years of being off-duty (I am prone to insomnia).
I also started avoiding Folic acid in favor of methylfolate. This is hard if you eat a lot of processed food, since they add folic acid to all kinds of things. Folic acid will block the absorption of methylfolate.
Edited by zrbarnes, 27 March 2012 - 08:54 PM.