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With intramuscular injections of B12, any excess will result in high levels within your blood so you may experience side effects such as headache, dizziness, tiredness, or gastrointestinal issues including nausea, until your body has metabolised any excess through your kidneys enroute to excretion via urine

Local rectal delivery in a gastrointestinal condition cannot be assumed to predict systemic exposure, tissue concentrations, effectiveness, or safety from an injection or oral product

Other options may be the lower arm deltoid muscle as well as the dorsogluteal site below the arm

This question requires medical guidance specific to your situation