Quoted from
Vitamin B12 in Algae: Spirulina, Chlorella and Nori | Dr. Schweikart
For a long time, spirulina was considered a vitamin B12 source. However, the B12 content in spirulina, which was originally measured by outdated methods, soon turned out to be B12 analogues in various studies (1-3). In fact, the B12 content has been measured to total at just 30%, a very low amount compared to the level of analogues (4, 5).
As expected, taking spirulina significantly worsened the B12 status of a research group in a study, which found that the analogues impaired the absorption rate of the genuine vitamin (6). Vegans, vegetarians and people with B12 absorption disorders should thus steer clear of spirulina products.
It is also not recommended to take spirulina alongside B12 supplements, as the analogues in the spirulina can hinder B12 absorption. Spirulina products are, for this reason, no longer allowed to be advertised as B12 sources (7).