You can consider trying a switch to a sublingual methylcobalamin and adenosylcobalamin combo like a seeking health or a drop form, and breaking it up so you dont take ass much, which would also help if there was anything allergic wise related to the shot like learner1 mentioned.
However I just noticed that B12 can lower Zinc, Vitamin A (two nutrients for eyes), Magnesium, Manganese and Chromium in a quick look.
https://traceelements.com/wheelindex.aspx Zinc can lead to a lowering of B6.
Note that DAO is involved with histamine break-down, and those nutrients are needed for the enzymes that break down histamine. So you can first try some retinol Vitamin A and add zinc if that doesnt help alone to start.
DAO appears to require many nutrients, so I can see this problem being common even without genetic issues. Copper, whole food vitamin C, magnesium and active vitamin B6 (p5p) not pyridoxine which lowers active B6, b12, iron, zinc all appear to be involved. Usable copper requires ceruloplasmin which if tests low may indicate problems with vitamin A, zinc, magnesium, boron. B6 needs zinc, magnesium, b2 to convert to P5P.
https://suzycohen.com/articles/histamine_intolerance_dao_genes_hashimotos/
Taking DAO supplements or Ancestral Supplements Kidney will only affect the amount of histamine in the food and beverage you consume after taking the supplement and this helps in decreasing the total level of histamine within the body (endogenous plus exogenous histamine) by removing the added exogenous food/drink source.
http://www.thedailyheadache.com/2014/05/testing-if-your-dao-level-is-low.html When histamine levels inside the body become excessive, DAO, and another enzyme called histamine N-methyl transferase (HNMT) which needs B2 break down the excess and mutations there can cause problems.
It is very important to understand that supplemental DAO and a histamine-restricted diet will not reduce the amount of histamine produced within the body. Some supplements I've taken that I researched and found worked on mast cells to reduce those body levels when lowering food sources and diet arent enough are palmitoylethanolamide (PEA), quercetin, pycnogenol (or the cheaper vitacost pine bark extract), stinging nettle root extract, zinc, vitamin d, source naturals SOD power as well as EGCG work on areas of histamine. Should be noted that taking too much can over inhibit these areas and lower stomach acid. Antihistamine drugs are also over the counter.