The oral administration of B12 (in pill form) carries significant drawbacks: Vitamin B12 is absorbed in the terminal ileum

Additional caution or monitoring may be appropriate in research settings involving: Pregnancy- or lactation-related research models, where limited safety data is available Active cancer-, tumor-, or abnormal proliferative research models, where NAD-related and metabolic-signaling pathways may influence cellular-signaling mechanisms Cardiovascular- or uncontrolled-hypertension research models, where metabolic-pathway modulation may influence cardiovascular-related biomarkers in some experimental settings Thyroid-related or endocrine research models, where metabolic-pathway investigations may influence endocrine-related observations Diabetes- or glucose-regulation research models, where insulin- and glucose-related pathways may warrant closer monitoring in experimental settings Moderate to severe hepatic- or renal-impairment research models, where compound metabolism or clearance mechanisms may be altered Autoimmune- or inflammatory-response research models, where immune-metabolic signaling pathways may influence inflammatory-related biomarkers or cytokine observations Research conditions, dosage ranges, administration frequency, protocol duration, and concurrent compounds may influence tolerability observations across experimental settings

American Journal of Cardiology, 6(1), 35-44
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Reported side effects include mild headaches or dizziness (more common when combined with growth hormone peptides), injection site irritation, GI discomfort from low-quality oral preparations, and occasional mood or energy changes