Much has been said that confuses the issues here. Let's not forget the basic reason most of us are doing this, WE WANT TO HEAL. We want to have decent health and the energy and ability to live our lives. Nobody here wants permanent damage that they must "LEARN TO LIVE WITH", that cursed phrase that so many of us have been told, "You'll just have to learn to live with it." I'm here because I refused to "learn to live with it" I refused to accept the idea that there was nothing but expensive drugs with terrible side effects to help a few out of hundreds of symptoms. I've been at this for 8 years and 1 month. I had 200+ symptoms from the list at that time and everything was getting worse. I was fading fast. Now, the only symptoms I have left are from probably permanent neurological damage. If I knew THEN what I know now I may not have any permanent neurological damage remaining. My WORST neurological damage didn't occur until I was 5 years into otherwise healing with mb12/adb12. My peripheral neuropathies and body based poly neuropathies were healed. My central neuropathy, subacute combined degeneration caused, was worse. I had no balance left, I had drop foot, my reflexes varied from none to "brisk" (highly variable, normal for b12 deficiency), I could not feel where my legs or feet were. I tripped on my toes hanging down, I stubbed toes and broke one and couldn't feel it generally. I was falling when I would go to strand up and couldn't tell where my feet and lower legs were, literally. Sometimes I would loose control of the muscles in my legs and drop to the ground unexpectedly and suddenly. This central neurological damage didn't start to heal or even slow down until until I went to injections of mb12 >=7.5mg. The setback with glutathione was so severe I have never fully recovered from it. Each episode of induced folate deficiency causes a setback. Recovery from each setback has not been as full as it was before. THE CLOCK IS TICKING from the moment b12 deficiency starts. Whether it is "fundamental" (absolute) or "functional" does not matter one hoot. That distinction is dangerous obfuscation of the reality because it doesn't matter. Twenty years of low CSF cobalamin and elevated CSF MMA, a "functional" deficiency, causing mitochondria malfunction and neurological damage results in Parkinson's. A decade or two of low CSF cobalamin and elevated homocysteine causes demyelination of the CNS. If it is bilateral it is diagnosed as subacute combined degeneration and if it isn't, MS. A decade or two of low CSF cobalamin with elevated CSF MMA and homocysteine and the damage done is called ALS. Early on low CSF cobalamin along with "functional" body deficiencies is called ME, CSF and FMS. When damage occurs many other symptoms occur that are diagnosed as any number of diseases. In postings and private communications from all sorts of people on this board and others, it is obvious that DAMAGE is occurring. Elevated homocysteine is associated with heart disease, strokes and a whole lot more. Elevated homocysteine is a result of low mb12 and/or methylfolate and/or P5P. Folate deficiency causes edema which can lead to congestive heart failure. Whenever I go into an induced folate deficiency I start accumulating 2-3 pounds of water per day, even with the diuretic. Then when the, in my case, paradoxical folate deficiency is overcome, the diuretic I take starts flushing out water and potassium. No wonder being a vegetarian caused congestive heart failure for me. Don't kid yourself. "Functional" b12 deficiencies can kill you. "Functional" b12 deficiencies can cause damage. B12 gets put back into circulation on the way to death by the "stores" in the liver being released into blood circulation by liver damage. The "stores" of b12 in the muscles are released into blood circulation by muscular atrophy. Both of these are on the way to death and only happen when the body is severely damaged. Such is the mythology of b12 that believes in some "stores" of b12 in the body. I have never had anything other than a "functional" b12 deficiency. When one has the low CSF/CNS cobalamin as is found in FMS, CFS, ALS, Parkinson's, Alzheimer's and others, nerve damage will occur. As one of the ads for Lyrica says, the nervous system becomes hyperactive for pain or however they say it. This is an early stage of damage. In my experience of healing and observing thousands of others improving and/or healing for those with low CSF/CNS cobalamin levels or conditions that research has shown to have said low CSF/CNS levels, hydroxycbl and cyanocbl don't have a snowflakes chance in hell of even stopping further damage much less reversing it. Researchers using methylb12 have commented over and over that methylb12 has neurological healing characteristics unmatched by hydroxycbl and/or cyanocbl. All we can do is play the odds. There are no certainties. Use of hydroxcbl which does NOT put enough mb12 and/or adb12 into the CNS to stop the continued damage to the CNS or to reverse it while the damage is still reversible is a dangerous and misguided idea. It might have it's place in the system as a whole as one of the MANY special purpose cobalamins that the body normally makes from adb12 and mb12. They are just beginning to catalog these and are 50 years or more away from understanding what the dozen or more possibly special purpose cobalamins do in the body. Hydroxycbl does appear to handle NO better than others. However, that is the only thing known that it does do better. The naturally occurring cyanocbl in the body is the end result of cyanide detox and is especially quickly excreted by the kidneys as is the glutathionylcbl, at least in the presence of the folate deficiency caused by the glutathione. The clock is ticking on damage. For a person with low CSF/CNS cobalamin levels despite normal or high serum levels such as is seen in CFS, FMS, ALS, MS, Parkinson's, ALzheimer's and others each day that passes without sufficienctly high mb12/adb12 levels to penetrate the CNS allows the damage to continue increasing and increases the likelihood that more of the damage is more severe and is permanent. The same is true on peripheral neurological damage which is not receiving sufficient mb12/adb12 for healing which might be possible but very slowly with hydroxycbl and/or cyanocbl. This doesn't even begin to address the damage to the epithelium (lungs, digestive system, bladder, skin, vaginal and cervix), endothelium (veins, arteries, lining of heart), muscles, various cells that become cancerous because of b12/mfolate lack causing faulty cell reproduction. As hydroxycbl results in 1/3 of those deficient not being helped at all for any given studied set of symptoms and for 2/3s of people leaving 2/3s of symptoms untouched, that means that 7/9s of the "functional" b12 deficiency symptoms are not fixed and the damage being done continues onwards until corrected with mb12/adb12. This is something you will find me commenting on in a variety of older posts as one of the most outstanding differences between the popluation here and at WD. As to serious side effect of methylcobalamin usage, the only one I can come up with is hypokalemia that can cause a lot of problems including paralysis of the ileum, heart beat irregularities and even death. Treatment of just about anybody with functional deficiencies of methylb12 is like to cause some degree of low potassium symptoms, whether it dips below the "lower limit" of 3.5-4.0 or not. I start having symptoms at 4.2 as do some others. As "functional" deficiencies can cause serious and permanent damage and even death I fail to see what difference being "functional" versus "absolute" based on a faulty understanding of what various tests may mean makes any difference at all. For everybody with an mb12, adb12 and/or methylfolate deficiency the clock is counting down to when any specific piece of damage is permanent. It might be a day or a week or a month or a year of delay that makes a difference. And there can be thousands of clocks each counting down to a specific piece of damage to become permanent. Based on research based on inactive cobalamins, hydroxcbl and/or cyanocbl, the time specified is within months of the first appearance of the neurological symptoms. Based on my experience with active b12s, I would be inclined to say months to years depending upon exact symptoms and how severe the deficiencies when one is working with methylb12, adenosylb12 and Metafolin.