Vitamin B12 (cobalamin) is an essential water-soluble vitamin vital for red blood cell formation, neurological function, and DNA synthesis. Whilst B12 deficiency requires treatment, many patients wonder: can B12 cause diarrhoea? Diarrhoea is an uncommon side effect of vitamin B12 supplementation, listed as a rare adverse reaction in UK product information for both oral and injectable preparations. Most people tolerate B12 well, even at high doses, as excess amounts are excreted through urine. When gastrointestinal symptoms occur, they often relate to supplement formulation, excipients, or underlying health conditions rather than the vitamin itself.

Can Vitamin B12 Cause Diarrhoea?

Vitamin B12 (cobalamin) is an essential water-soluble vitamin crucial for red blood cell formation, neurological function, and DNA synthesis. Whilst B12 deficiency can cause serious health problems, supplementation is generally well-tolerated by most individuals. Diarrhoea (defined as three or more loose or watery stools per day) is an uncommon side effect of vitamin B12 supplementation, though it is listed as a rare or uncommon adverse reaction in the official product information for both oral cyanocobalamin and injectable hydroxocobalamin preparations.

The vitamin's water-soluble nature means that excess amounts are typically excreted through urine rather than accumulating in the body to toxic levels. This characteristic makes B12 one of the safer vitamins to supplement, with no established upper tolerable intake level set by UK health authorities. Most people tolerate B12 supplementation well, even at doses significantly higher than the recommended daily amount.

However, gastrointestinal disturbances can occasionally occur, particularly with certain formulations. The relationship between B12 and bowel changes is typically indirect, relating more to the supplement formulation, excipients (inactive ingredients), or underlying health conditions rather than the vitamin itself. Understanding this distinction is important for patients experiencing digestive symptoms whilst taking B12 supplements.

If you develop diarrhoea after starting B12 supplementation, it is worth considering other factors that may be contributing to your symptoms. The timing, severity, and accompanying symptoms can provide valuable clues about whether the supplement is genuinely responsible or whether another cause should be investigated.

Why B12 Supplements May Affect Your Bowels

When gastrointestinal symptoms occur with B12 supplementation, several factors may be responsible. The formulation and excipients in B12 supplements often play a significant role. Many commercial preparations contain additional ingredients such as:

• Sorbitol or mannitol – sugar alcohols used as sweeteners in some tablets that can cause osmotic diarrhoea when consumed in larger quantities, though the amounts in typical supplements are usually too small to cause symptoms alone

• Binding agents and fillers – including lactose, which may cause symptoms in people with lactose intolerance, though tablet quantities are typically small

• Other additives – various excipients that some individuals may be sensitive to

The dose and frequency of supplementation may also influence tolerability. Very high doses of B12 taken orally may occasionally cause mild gastrointestinal disturbance in some individuals, though this is uncommon. Sublingual formulations, which dissolve under the tongue, are often marketed as superior to standard tablets, but most of the dissolved vitamin is actually swallowed with saliva and absorbed through the digestive tract similarly to regular oral supplements.

Intramuscular B12 injections (hydroxocobalamin or cyanocobalamin) bypass the digestive system, but UK product information (SmPCs) for hydroxocobalamin injections does list gastrointestinal disturbances, including diarrhoea, as possible adverse reactions, albeit rare. Some patients report temporary digestive changes following injections, though the exact mechanism for this is not well established.

In individuals with pre-existing gastrointestinal conditions such as inflammatory bowel disease, coeliac disease, or irritable bowel syndrome, introducing any new supplement may temporarily affect bowel habits as the digestive system adapts. This is not specific to B12 but reflects the sensitivity of the gastrointestinal tract in these conditions.

Other Causes of Diarrhoea When Taking B12

When diarrhoea develops after starting B12 supplementation, it is essential to consider alternative explanations that may coincide with, rather than result from, the vitamin therapy. Several important factors warrant consideration:

Concurrent medications and supplements frequently contribute to gastrointestinal symptoms. Metformin, commonly prescribed for type 2 diabetes, is well-known to cause diarrhoea and is also associated with B12 deficiency—meaning patients may be taking both simultaneously. Proton pump inhibitors (PPIs) such as omeprazole can reduce B12 absorption and may cause digestive upset. Antibiotics can disrupt gut flora and occasionally lead to Clostridioides difficile infection. Other supplements taken alongside B12, particularly magnesium, vitamin C in high doses, or probiotic preparations, may affect bowel habits.

Underlying gastrointestinal conditions should be investigated if diarrhoea persists. Pernicious anaemia (an autoimmune condition affecting B12 absorption) is a common cause of B12 deficiency that requires specific testing for anti-intrinsic factor antibodies. Coeliac disease, inflammatory bowel disease (Crohn's disease or ulcerative colitis), and chronic pancreatitis can all cause both B12 malabsorption and diarrhoea. Bacterial overgrowth in the small intestine may similarly present with both symptoms. The presence of B12 deficiency itself may indicate an underlying digestive disorder that requires specific investigation and management.

Dietary changes often accompany the decision to supplement with B12. Individuals adopting plant-based diets may simultaneously increase their fibre intake or consume more legumes, both of which can temporarily affect bowel habits. Food intolerances, particularly to lactose or fructose, may become more apparent during dietary transitions.

Infections and acute illnesses, including viral gastroenteritis, food poisoning, or antibiotic-associated diarrhoea, may coincidentally occur when someone has recently started B12 supplementation. The temporal association does not necessarily indicate causation. If diarrhoea is accompanied by fever, blood in stools, severe abdominal pain, or dehydration, an infectious or inflammatory cause should be considered and medical attention sought promptly.

When to Seek Medical Advice About B12 Side Effects

Whilst B12 supplementation is generally safe, certain symptoms warrant prompt medical evaluation. You should contact your GP or NHS 111 if you experience:

• Persistent diarrhoea lasting more than 7 days in otherwise healthy adults

• Blood or mucus in stools, which may indicate inflammatory bowel disease or infection

• Severe abdominal pain or cramping that is worsening or different from previous digestive symptoms

• Unexplained weight loss alongside bowel changes

• Signs of allergic reaction, including rash, itching, swelling, or difficulty breathing (call 999 or go to A&E for severe reactions)

• Neurological symptoms such as numbness, tingling, balance problems, or memory difficulties, which may indicate B12 deficiency is not being adequately treated

Seek help sooner if you have red flag symptoms, are pregnant, immunosuppressed, elderly, or have significant underlying health conditions.

Before starting B12 supplementation, discuss with your GP if you have existing gastrointestinal conditions or are taking multiple medications. Your doctor can recommend the most appropriate formulation and dose for your circumstances. NICE guidance emphasises the importance of investigating the underlying cause of B12 deficiency rather than simply treating with supplements, as this may identify serious conditions requiring specific management. Tests may include full blood count, serum B12 levels, anti-intrinsic factor antibodies (to diagnose pernicious anaemia), and sometimes methylmalonic acid or homocysteine levels when the diagnosis is uncertain.

If you suspect your B12 supplement is causing diarrhoea, consider trying an alternative formulation or a different brand with fewer excipients. Taking the supplement with food may improve tolerance. However, do not discontinue prescribed B12 treatment without medical advice, particularly if you have pernicious anaemia or confirmed deficiency, as untreated B12 deficiency can lead to irreversible neurological damage.

If you believe you've experienced side effects from vitamin B12 supplementation, you can report this through the MHRA Yellow Card Scheme (yellowcard.mhra.gov.uk or via the Yellow Card app), which helps monitor the safety of medicines and supplements in the UK.

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