Back to the original question - I would like to establish that "methylfolate" side effects are actually "methylation startup" side effects. Apparent B12 side effects would be the same as apparent methylfolate side effects. It just depends on which one you started first and which one you started second as to what you thought was giving you the side effects.
But in reality, it's the combination of the two. For example, as a kid you may have played around with making a "volcano", which was the reaction between baking soda and vinegar. If you just have vinegar, nothing happens. If you just have baking soda, nothing happens. You put the two together, and you get the foaming/fizzing reaction.
So you can start with some baking soda first and add vinegar second, and get the same reaction as if you started with vinegar first, then added baking soda.
This is similar to folate and B12. If you started with B12 and tolerated it, then added methylfolate and got side effects, the side effects are not from folate. Similarly, if you started with folate and tolerated it, and added B12 and got side effects, it's not from the B12.
It's from the combination of the two. When you combine the two, you crank up methylation to a much higher level than it was. Methylation affects over 40 reactions in the body. Making neurotransmitters and detoxifying are two examples. If all of these things are suddenly turned back on, your body has a tremendous amount of work to do with rebuilding, repairing, detoxifying, etc.There will be various nutrients that your body will need to assist with this. You may be deficient in these, or even if your levels are good, your body might require more of them for a period of time. If your body requires more for methylation, it might steal it away from another area to get it. All of these things can cause side effects.
Next post - what to do.