It can range from 250-500 IU for testicular function to higher doses for fertility
Natl Acad
While individual comfort levels vary, B12 injections are easily tolerated by most patients
For example: Fatigue, unexplained weight loss, or abdominal pain may indicate liver disease or cancer Easy bruising, night sweats, or feeling full quickly (early satiety) may suggest a myeloproliferative disorder Swelling in the legs, reduced urine output, or persistent nausea may point to kidney disease In cases of very high B12 from injections (not oral supplements), some people have reported headache, nausea, diarrhea, facial flushing, or skin rash
10.5009/gnl18490 60 SikiricP.RucmanR.TurkovicB.SeverM.KlicekR.RadicB.et al (2018)
Oral administration floods the gut lumen and intestinal lining with peptide, concentrating action locally