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Methalfa should be taken with food, since it can cause cause digestive issues gor some. Methalfa is a combination of methylcobalamine with alpha lipoic acid and other B vitamins.
Whereas other supplements may only have methylcobalanine or cyanocobalamine. The dosage would however depend on your blood test levels.
Health Tips
Here is an informative blog on how to get adequate
vitamin B12 on a plant based diet-
https://www.pragmaticnutritionist.com/post/vitamin-b12-rich-foods-vegetarian