> The paradoxical B12 deficiency might have a relation with the inactive form being supplemented.

Yup, not everyone can convert cyanocobalamin or hydroxocobalamin to adenosyl-/methylcobalamin. Especially in severe cases, anecdotally I’ve seen people not make progress with the standard cyanocobalamin injections, but then make huge progress with methylcobalamin injections. Unfortunately, methylcobalamin is often not preferred in injectable form due to very quick degradation into hydroxocobalamin upon exposure to light.

Taking methylcobalamin B12 supplements essentially fixed my memory problems. Early on I tried out the cyanocobalamin type and got absolutely nothing from it.

If I stop taking the supplements for a few weeks, the memory problems come back. If I start again, a week later my memory is excellent. It’s well worth the minor effort.

Subligual methylcobalamin works just as well as the shots and you can buy it anywhere.

Also, get a Methylmalonic Acid Test with you B12 levels. An MMA test is more useful because it is an enz=yme which uses B12, so when the active form of B12 is low, so will MMA in most cases.