Nothing detectable happens on day 29, which is exactly why the question keeps getting asked
Regul Pept 181: 5066
Vitamin B12 is crucial for several physiological processes, including: Red blood cell formation deficiency leads to megaloblastic anaemia, characterised by large, immature red blood cells Neurological function B12 is required for myelin synthesis and nerve health DNA synthesis essential for cell division and growth Without adequate B12, patients may develop symptoms such as fatigue, pallor, shortness of breath, glossitis (sore tongue), paraesthesia (pins and needles), and in severe cases, neurological complications including peripheral neuropathy, ataxia, and cognitive impairment
inject slowly and steadily
Kim HW, Rapoport SI, Rao JS