Hi Francelle, how many people get follow-up levels of B12 done after they have been on it for awhile? Are measurements even useful? Mine was never measured and would be pretty pointless now. At my previous dose of 10mg injected SC 3x per day an estimate of serum level is about 200,000pg/ml. At 10mg x3 per day I get sustained healing activity in the CNS. Twice a day allows fallback. I am doing higher doses currently as a trial. To pay a sizable price to be told my serum cobalamin level is >1100pg/ml is rather pointless. Do various forms of administration, such as those being discusssed in this thread, produce better serum/blood levels? A mg put into serum injected IM would produce an instantaneous serum level of about 200,000 pg/ml decreasing by 75% each hour for several hours and start slowing down after that. An SC injection takes up to 10 hours or so to largely enter serum. A 5mg Jarrow held for 2 hours also puts about 1mg into serum over 2 hours so the peak isn't as high but lasts longer. Other methods could likely achieve something similar. I did a urine colorimetry set of tests of sublingual compared to SC injection in order to come up with the sublingual absorption rate. What about intracellular levels? High serum levels well in excess of what can be carried by HTC2 allows all tissues to be saturated via diffusion. Even the CNS/CSF is penetrated in the 100,000pg/ml to 200,000pg/ml range or more. Why do some people continue to take mega-doses when their levels are elevated or high in range? The serum halflife of high levels is about 30 minutes until it gets down to an estimated 25,000pg/ml or thereabouts and then starts falling off. It takes high levels to provoke CNS healing especially in some disorders such as FMS/CFS where the CSF has low serum level despite relatively high serum levels. In other words it works. Do blood levels belie effectiveness or otherwise of the B12? I don't know what you are asking. Is it better to take several types of B12 such as methyl, cyan, hydroxy - rather than just one type of cobalamin? It is more effective on more symptoms to take mb12 and adb12, the ONLY 2 active forms in the human body. All others are temporary but inactive forms and cyanocbl is post cyanide detox waste form that is excreted especially rapidly.