As vegans, we need B-12 supplementation.
The linked study shows excess B-12 causes excess death in all demographics. Most supplements out there are way more than your RDA often like 200x or more. Despite the claims that because it is water soluble, it it non-toxic, it still is known to cause toxicity. It is stored in your liver for years and high circulating blood levels can cause cobalamin toxicity.
Just like years ago we found out about B9 (as folic acid) toxicity and birth defects, then B6 toxicity, which is now making news and finally being regulated. These cause the same symptoms at B12 toxicity, of which we are beginning to see more studies. Even those all these B vitamins are are water-soluble and we were told not to worry, they can all cause toxicity.
Here is another study showing excess B6 and/or B12 leads to increase incidence of hip fracture in post-menopausal women: https://pubmed.ncbi.nlm.nih.gov/31074816/.
What I wish someone would have told me: While taking a B12 supplement, If you are experiencing excessive flushing, restlessness, muscle spasms and tingling in the hands in the feet (that is neuropathy and nerve damage is irreversible), consider the possibility of your B12 intake causing toxicity, especially if you take high doses regularly. Some bodies are more prone to it than others.
Stay healthy friends. <3
A single person case study:
I'm not properly vegan, I drink "kefir" (fermented milk product), but it's not a source of B12.
I put flavour yeast / nutritional yeast on many foods, typically a large spoonful per day, sometimes two.
Nutritional yeast naturally contains other B-group vitamins, B12 has been artificially added from bacterial sources.
My preferred product (products vary) contains 2.2 micrograms of B12 per 5 grams, which is claimed to be 88% of the recommended daily amount. If I consume 1.5 spoonfuls per day, I'm consuming approximately 1.5 RDA per day.
According to local sources, B12 levels in blood are recommended to be within 156 - 672 picomoles per liter. My samples have yielded values like 230, 190, 255, 208, etc.
Conclusion: in my case, eating 1.5 x RDA per day is safe and arguably less than optimal, I should eat a bit more. But definitely don't eat 200 x RDA of flavour yeast, that would be asking for trouble (disclaimer: I studied biology but didn't become a biologist).