The type of b-12 that works the best is hydroxocobalamin (as an injection is better than tablet) https://b12researchgroup.wordpress.com/warning-about-the-use-of-b12-sublingual-tablets/
Often more frequent hydroxocobalamin is a good solution, as is the case in very serious metabolism disorders. In a few cases we try methylcobalamin. In the methylation process, other co-factors (like folate) are very important too. There could also be other underlying factors causing remaining symptoms such as: thyroid problems, Lyme disease, copper deficiency and an excess of B6 supplementation.
