Thought I would pass this on, since I know many people here end up taking B12, folate, etc, and sometimes without the support of a doctor. I didn't know all the details of this, and I could see how it could be dangerous to not have this information. I preface all of this by saying that I am not a doctor, so it isn't my intent to give medical advice, etc., etc. Obviously discussing with a knowledgeable doctor would be the ideal course of action. Basically, if you have a folate deficiency, you must make sure your B12 levels are okay before you treat it. And just methyl B12 may not be sufficient. (I hate quoting wikipedia, but this was the first relevant quote I came across, and I didn't have the energy to search for a more credible example). http://en.wikipedia.org/wiki/Subacute_combined_degeneration_of_spinal_cord If you have both a B12 and a folate deficiency, it is important to get your B12 up to good levels before you introduce folate into the picture. And it seems potentially important that it is not just methyl B12 (which is what so many of us take). This is the part that seems key, but that I am also less clear on the details of. Some countries use Hydroxy B12, most use Cyano B12 (which I know we all hate), so both of those seem fine. The problem seems to be that the body prioritizes certain functions of B12 over others. And when you've been deficient in both and add folate, it will turn what B12 you have into methyl B12 (for use in making blood cells with the folate, I assume). BUT...there may not be sufficient B12 supplies left over for the other things it needs to do. Methyl B12 is great at what it does, and is a critical thing for us, but it doesn't do everything. And that can result in an even worse B12 deficiency for some body parts and resulting damage to the nervous system that can be permanent. So you would first need to take enough Hydroxy B12 to get your levels up, THEN introduce the folate. As I understand it, taking methyl B12 in addition should be fine, but methyl B12 on its own may not prevent the damage. Hydroxy (or Cyano) before the folate seems key. Note that this should only be a problem if you are deficient in both, but it seems like some of us have been so I thought it was worth mentioning. If a doctor catches it, they'd give you a shot of B12 and everything would be fine, but I know that not everybody here has a doctor working with them on this stuff. And since we spend a lot of time talking about functional B12 and folate deficiency, it might be worth taking some extra hydroxy B12 before the folate just in case even if there isn't an obvious enough problem for the doctors to easily catch (since the whole point of us taking B12 and folate in the first place is the assumption that we are deficient in ways that may not show up on those tests). Like many things, some of the symptoms of nervous system damage caused by this are things that are so inherent in this illness that we might never notice them, or think that something we were doing could be causing them (fatigue, muscle weakness, sleepiness, balance problems, tingling, depression, etc.). Anyway, just thought it might be worth mentioning. I'm not 100% certain on the methyl B12 issue myself, but that seems to be the implication. If anyone can clarify, or has spoken with a doctor about treating folate deficiency in the presence of B12 issues, please feel free to weigh in. Use the information as you will.