These may include: Nausea and stomach upset Diarrhea Constipation Injection site redness or swelling Itching Headache Dizziness Blood clots Swelling In rare cases, some people may experience an allergic reaction to vitamin B12, which can cause symptoms such as hives, difficulty breathing, and swelling of the face, lips, tongue, or throat

For patients seeking weight loss who request B12 injections, GPs should: Assess for genuine deficiency through appropriate blood tests rather than empirical treatment Provide education about the lack of evidence supporting B12 for weight loss in non-deficient individuals Explore underlying concerns about weight and energy levels, addressing these through appropriate channels Signpost to evidence-based resources such as NHS weight management services according to local pathways Patients should contact their GP if they experience unexplained fatigue, neurological symptoms (numbness, tingling, balance problems), or signs of anaemia (pallor, breathlessness, palpitations), as these may indicate B12 deficiency requiring investigation
We cover the research, practical guidance, and what to discuss with your physician
Lifestyle factors greatly influence ones Vitamin B levels