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Wegovy (semaglutide 2.4 mg) is a GLP-1 receptor agonist licensed in the UK for chronic weight management in adults with obesity or overweight with comorbidities. As its use expands within specialist tier-3 services, questions have emerged about potential interactions with vitamin B12, an essential nutrient critical for red blood cell formation, neurological function, and energy metabolism. Whilst Wegovy does not directly interfere with B12 absorption or metabolism, its effects on appetite and food intake may indirectly influence nutritional status. This article examines the evidence on Wegovy and B12, clarifies the risks, and provides practical guidance for monitoring and managing B12 levels during weight management treatment.
Quick Answer: Wegovy (semaglutide) does not directly cause vitamin B12 deficiency, but reduced food intake may indirectly lower dietary B12 consumption in some patients.
Wegovy (semaglutide 2.4 mg) is a glucagon-like peptide-1 (GLP-1) receptor agonist licensed by the MHRA for chronic weight management in adults with obesity (BMI ≥30 kg/m²) or overweight (BMI ≥27 kg/m²) with at least one weight-related comorbidity. In the UK, it should be used as part of a specialist tier-3 weight management service. Wegovy works by mimicking the natural hormone GLP-1, which regulates appetite and food intake by acting on areas of the brain that control hunger. Wegovy transiently slows gastric emptying, increases feelings of fullness, and reduces appetite, leading to decreased caloric intake and subsequent weight loss.
Vitamin B12 (cobalamin) is an essential water-soluble vitamin crucial for numerous bodily functions, including red blood cell formation, DNA synthesis, neurological function, and energy metabolism. The body cannot produce B12, so it must be obtained through diet—primarily from animal products such as meat, fish, dairy, and eggs—or supplements. B12 absorption occurs in the terminal ileum and requires intrinsic factor, a protein produced by the stomach's parietal cells.
As Wegovy gains widespread use for weight management, patients and healthcare professionals have raised questions about potential interactions between this medication and vitamin B12 levels. Understanding the relationship between GLP-1 receptor agonists and micronutrient status is important for optimising patient outcomes and preventing nutritional deficiencies during weight loss treatment. This article examines the current evidence regarding Wegovy's potential effects on B12 levels and provides practical guidance for monitoring and management.
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Currently, there is no established direct pharmacological link between Wegovy (semaglutide) and vitamin B12 deficiency. The mechanism of action of GLP-1 receptor agonists does not directly interfere with B12 absorption, intrinsic factor production, or the vitamin's metabolic pathways. Clinical trials of semaglutide have not identified B12 deficiency as a recognised adverse effect, and it is not listed among the common or uncommon side effects in the Summary of Product Characteristics (SmPC) approved by the MHRA.
However, indirect effects warrant consideration. Wegovy significantly reduces food intake and alters eating patterns, which may lead to decreased dietary B12 consumption, particularly if patients reduce their intake of animal products. The medication's transient effect on gastric emptying theoretically could affect the initial stages of B12 release from food proteins, though this remains speculative and has not been demonstrated in clinical studies.
It is important to distinguish Wegovy from metformin, a diabetes medication with well-documented effects on B12 absorption. Metformin interferes with calcium-dependent B12-intrinsic factor uptake in the terminal ileum, and long-term use is associated with reduced B12 levels in approximately 10–30% of patients, as noted in NICE guideline NG28 for Type 2 diabetes management. Some individuals taking Wegovy may also be prescribed metformin for type 2 diabetes management, which could confound the picture. In such cases, any B12 deficiency is more likely attributable to metformin rather than semaglutide. Healthcare professionals should consider all medications when assessing B12 status in patients on combination therapy.
Vitamin B12 deficiency develops gradually, as the liver stores sufficient B12 to last several years. Early symptoms may be subtle and non-specific, making recognition challenging. Common manifestations include persistent fatigue and weakness, which patients may mistakenly attribute to caloric restriction or weight loss itself. Other early signs include pale or jaundiced skin, glossitis (a sore, red tongue), and mouth ulcers.
Neurological symptoms are particularly concerning and may include paraesthesia (pins and needles) in the hands and feet, difficulty walking or balance problems, memory difficulties, confusion, and mood changes including depression or irritability. In severe or prolonged deficiency, irreversible neurological damage can occur, emphasising the importance of early detection. Patients with neurological symptoms require urgent referral to neurology or haematology services. Patients may also experience breathlessness, dizziness, and heart palpitations due to anaemia.
Gastrointestinal symptoms such as diarrhoea, constipation, loss of appetite, and flatulence may occur, though these can overlap with common side effects of Wegovy itself, potentially masking B12 deficiency. The most common adverse effects of Wegovy include nausea, vomiting, diarrhoea, and constipation, which affect a significant proportion of users, particularly during dose escalation. Patients should report suspected adverse reactions to Wegovy via the MHRA Yellow Card Scheme.
Patients taking Wegovy should be advised to contact their GP if they experience persistent unexplained fatigue, neurological symptoms such as numbness or tingling, significant mood changes, or a sore tongue that does not resolve. Healthcare professionals should maintain a low threshold for investigating B12 status in patients presenting with these symptoms, particularly those with additional risk factors such as vegan or vegetarian diets, older age, or concurrent metformin use.
Routine B12 supplementation is not currently recommended for all patients taking Wegovy, as there is no evidence that the medication directly causes B12 deficiency. Unnecessary supplementation may lead to patients overlooking the importance of a balanced diet and could potentially mask underlying conditions that cause B12 malabsorption, such as pernicious anaemia or gastrointestinal disorders.
However, targeted supplementation may be appropriate for certain patient groups. Individuals following vegan or strict vegetarian diets are at higher risk of B12 deficiency regardless of Wegovy use, as plant-based foods do not naturally contain adequate B12. These patients should consider B12 supplementation (typically 50 micrograms daily or 2,000 micrograms weekly) or ensure consumption of fortified foods, as recommended by the British Dietetic Association. Older adults (over 60 years) have reduced stomach acid production, which impairs B12 release from food, and may benefit from supplementation.
Patients taking metformin alongside Wegovy should be monitored more closely, and supplementation may be warranted if levels are suboptimal. NICE guideline NG28 on type 2 diabetes management recommends considering B12 monitoring in patients on long-term metformin therapy. Those with previous gastrointestinal surgery (particularly gastric bypass or ileal resection), pernicious anaemia, or conditions affecting absorption (such as Crohn's disease or coeliac disease) require regular monitoring and often need supplementation or intramuscular B12 injections.
Before starting supplements, patients should consult their GP or healthcare provider. Blood tests can establish baseline B12 status and guide appropriate management. Self-prescribing high-dose supplements without medical advice is not recommended, as excessive B12 intake, whilst generally considered safe, may interfere with certain laboratory tests and could potentially mask folate deficiency.
Baseline assessment before initiating Wegovy may include a nutritional evaluation, particularly for patients with risk factors for B12 deficiency. While not mandated by current NICE guidelines (NG246), measuring serum B12 levels may be considered in high-risk individuals, including those over 60, vegans or vegetarians, patients with gastrointestinal conditions, or those taking metformin. Normal serum B12 levels typically range from 200–900 ng/L (approximately 148–664 pmol/L), though reference ranges vary between laboratories and symptoms may occur even within the lower normal range.
Ongoing monitoring should be individualised based on risk factors and clinical presentation. For patients without specific risk factors, routine B12 testing is not necessary unless symptoms develop. However, those with pre-existing low-normal B12 levels, dietary restrictions, or concurrent medications affecting B12 should have levels rechecked at 6–12 month intervals. If B12 deficiency is suspected clinically but serum levels are borderline, additional tests such as methylmalonic acid (MMA) or homocysteine levels may provide greater diagnostic clarity, as these metabolites accumulate when B12 is functionally deficient. Concurrent folate testing is also recommended, as deficiencies often coexist.
Management strategies for confirmed B12 deficiency depend on severity and underlying cause. Mild deficiency in patients with adequate absorption may respond to oral supplementation (50–2,000 micrograms daily). More significant deficiency, particularly with neurological symptoms or malabsorption, requires intramuscular hydroxocobalamin injections, typically 1 mg administered three times weekly for two weeks, then maintenance doses every two to three months, as recommended in the British National Formulary (BNF).
Dietary optimisation remains fundamental. Healthcare professionals should provide nutritional counselling to patients on Wegovy, emphasising the importance of nutrient-dense foods despite reduced appetite. Encouraging consumption of B12-rich foods—including lean meats, fish, eggs, and dairy products, or fortified plant-based alternatives—supports adequate intake. Referral to a registered dietitian may benefit patients struggling to meet nutritional requirements whilst on appetite-suppressing medication. Regular follow-up allows early identification of deficiencies and ensures that weight loss is achieved safely without compromising nutritional status.
Wegovy does not directly cause B12 deficiency through its mechanism of action. However, reduced food intake may indirectly lower dietary B12 consumption, particularly in patients with pre-existing risk factors such as vegan diets or concurrent metformin use.
Patients at higher risk include those over 60 years, vegans or vegetarians, individuals taking metformin, and those with gastrointestinal conditions affecting absorption. Baseline and periodic monitoring at 6–12 month intervals may be appropriate for these groups.
Key symptoms include persistent fatigue, neurological signs such as paraesthesia or balance problems, glossitis (sore red tongue), mood changes, and anaemia-related symptoms like breathlessness. Patients experiencing these should contact their GP for assessment and possible B12 testing.
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