Methyl groups are flags that are used for many purposes in the body. They can be used to inactivate substances like estrogen, histamine, dopamine, viruses, some dietary amines as well as flag some toxins for disposal. This process can be blocked by some metals, including lead.
Starting the methyl cycle up after it's been 'off' for an extended period of time can release many toxins, creating a detox reaction. This may explain bad reactions to B12 and/or methyl folate.
Reasons to Suspect
Estrogen dominance, seasonal allergies, high histamine, amine sensitivity, frequent illness, high homocysteine.
Methionine from the diet is used to make methyl groups. After the methyl group has been used, it becomes homocysteine. With folate and B12, homocysteine can be recycled back into methionine to make more methyl groups.
There is a 'back door' way to recycle homocysteine, which uses betaine (TMG: tri methyl glycine) and is stimulated by phosphatidyl serine and phosphatidly choline. This pathway increases your norepinephrine, so might increase stress levels.