Vitamin B12 side effects: tablets, injections and what is normal

Vitamin B12, or cobalamin, is a water-soluble vitamin. It comes almost exclusively from animal foods: meat, fish, shellfish, eggs and dairy products. The recommended intake for adults is 2.4 micrograms a day, a tiny amount compared with the doses found in supplements, which commonly contain 500 to 1,000 micrograms per tablet. That gap is the reason so many people wonder whether they are taking too much, and whether the side effects they read about are real.

The short answer: vitamin B12 is one of the safest vitamins there is. The European Food Safety Authority (EFSA) has identified no defined adverse effects and has therefore set no upper limit, and the MSD Manual states that large amounts “seem to be nontoxic”. That does not mean nothing ever happens. A few effects are well described, most of them mild, and one of them, red urine after an injection, alarms people who were never told to expect it. Here is the full picture.

What are the side effects of vitamin B12?

For the vast majority of people, vitamin B12 is very well tolerated. A handful of effects, generally mild, can nevertheless occur:

• Red or pink urine: a normal and harmless phenomenon, linked to the red pigment of cobalamin, most visible after an injection;
• Acne or acne-like spots: occasionally reported with high-dose or prolonged use, and they usually disappear quickly once the supplement is stopped;
• Allergic reactions: itching, hives, eczema, and very rarely a severe reaction;
• Mild digestive upset with tablets: nausea, loose stools, usually transient;
• Trouble falling asleep when taken late in the day, reported by some people;
• Injection-site reactions: local pain, redness or skin irritation, sometimes mild dizziness or a hot flush right after the shot.

None of these is a reason to avoid B12 when you need it. They are reasons to take it at the right time of day, at the right dose, and to know what is normal.

Vitamin B12 tablets: side effects to expect

Oral B12 is sold in tablets, capsules, sublingual strips and sprays, most often at 500 or 1,000 micrograms. At these doses, only a small fraction is absorbed (a few percent), which is exactly why the doses are so high. The effects reported with tablets are mostly digestive: a queasy stomach, nausea, loose stools, especially when taken on an empty stomach. They are mild and tend to settle within days. Taking the tablet with food usually solves the problem.

Two things are worth knowing. First, many “B12” tablets are in fact B-complex products, and the effects people attribute to B12 (flushing, tingling, digestive upset) often come from the niacin or the vitamin B6 they also contain. Read the label, and see our article on the side effects of vitamin B. Second, the form matters little for safety: cyanocobalamin, methylcobalamin and hydroxocobalamin are all well tolerated by mouth.

Vitamin B12 injection side effects

Injections are used when absorption is the problem (pernicious anaemia, gastric surgery, certain bowel diseases, long-term antacid use) or when a deficiency is severe. In Switzerland, the standard injectable form is hydroxocobalamin (for example Vitarubin Depot), given into the muscle. The effects documented for hydroxocobalamin injections are:

• A temporary red colouring of the skin and urine, the most common effect and the one that worries people most;
• Local reactions at the injection site: pain, redness, skin irritation;
• Mild dizziness or hot flushes in the minutes after the injection;
• Acne outbreaks or acne-like spots with high-dose or prolonged injections, which usually clear quickly after stopping;
• Digestive discomfort, occasionally;
• Very rarely, an allergic reaction, which is why the first injection is always given by a healthcare professional.

Pain at the injection site is the most frequent complaint in practice and lasts a day or two. Everything else is uncommon. Women and men report the same effects: there is no documented sex-specific side effect of B12, only differences in needs, for example during pregnancy and breastfeeding.

Red urine after B12: why it is normal

Cobalamin owes its name and its colour to the cobalt atom at its centre: it is naturally deep red, which is why B12 tablets and ampoules are red, and why it is sometimes called “the red vitamin”. Whatever the body does not use is excreted in the urine, which can turn pink or red for a few hours after a tablet and for longer after an injection. Drug references list this temporary red colouring of skin and urine as the most common effect of hydroxocobalamin, and the official product information for high-dose hydroxocobalamin describes a dark red colouring of the urine in all patients. It is not blood, it is not a warning sign, and it needs no treatment. If the colour persists for more than a few days after a single tablet, or if you have not taken any B12, see a doctor, because red urine then has other possible causes.

Vitamin B12 and acne

The link between B12 and acne has been noted by doctors for decades, mainly with injections and very high doses. Drug references describe acne outbreaks or acne-like spots with high-dose or prolonged hydroxocobalamin use, and note that they usually disappear quickly once treatment stops. If you develop spots a few weeks after starting B12, talk to the prescribing doctor: reducing the dose or spacing the injections is often enough. Do not stop a prescribed treatment for a confirmed deficiency on your own.

Vitamin B12 and sleep: why to avoid it in the evening

Some people report difficulty falling asleep after taking vitamin B12, or more vivid dreams. The effect is poorly studied and does not occur in everyone, but it is easy to avoid: take your B12 in the morning or early afternoon rather than at night. For injections, the timing matters less, but the same advice applies if you notice a restless night after a shot.

Allergic reaction to vitamin B12

An allergic reaction can appear as itching, hives or eczema, and exceptionally as a severe reaction with swelling of the face or difficulty breathing. It is rare, and it can involve the cobalamin itself, the cobalt it contains, or an excipient in the product. Anyone with a known cobalt or cobalamin allergy should avoid B12 supplements and tell their doctor before any supplementation. Hives or facial swelling after a dose is a reason to stop and seek medical advice promptly.

Can you overdose on vitamin B12?

Not in any practical sense. EFSA lists vitamin B12 as having “no defined adverse effects” and has set no tolerable upper intake level, unlike vitamin B6 or niacin. The MSD Manual states that large amounts seem nontoxic, while adding that they are “not recommended for regular use”, because there is simply no benefit in taking more than you need. Taking 1,000 micrograms a day when your level is normal will not harm you, but it will not help you either.

One situation does deserve attention: a high blood B12 level in someone who takes no supplements. That is not an overdose, it is a laboratory finding that can point to a liver or blood condition, and it should always be reviewed by a doctor.

Vitamin B12 deficiency: the real issue to watch

If the side effects of B12 worry people, it is in fact deficiency that is common. According to the MSD Manual, inadequate absorption is the most frequent cause, ahead of a low dietary intake. The people most at risk are:

• Vegans and vegetarians, because B12 comes from animal foods;
• Adults over 60, whose stomach produces less acid and absorbs B12 less well;
• People with malabsorption: pernicious anaemia, gastric surgery, inflammatory bowel disease, bacterial overgrowth;
• People on long-term metformin or antacids (proton pump inhibitors).

A B12 deficiency can show as fatigue, tingling in the hands and feet, memory problems, pallor or irritability. The MSD Manual stresses that severe nerve damage can occur without anaemia, which is why symptoms alone are not enough to rule it out. For the wider picture, read our article on the most common deficiencies.

Measure your B12 at home in Switzerland

The only way to confirm a deficiency (or an excess) is a blood test. YuBoost’s vitamin and mineral check-up measures your vitamin B12 level along with 10 other biomarkers: vitamins B6, B9 and D, calcium, iron, ferritin, transferrin, magnesium and zinc. How it works:

• Blood draw at home by a registered nurse, no prescription needed, in Lausanne, Geneva, Zurich and across French-speaking Switzerland;
• Analysis in an accredited Swiss laboratory, results within 7 working days;
• Personalised medical report and recommendations, with an optional online consultation with a doctor.

The vitamin and mineral check-up costs CHF 370. If a deficiency is confirmed and tablets are not the right option, a vitamin B12 injection can be given at home on medical indication. A B12 injection is prescribed after a blood test, never blindly. To know when supplementation makes sense in the first place, see in what cases vitamin B12 should be taken.

Frequently asked questions

Sources

• MSD Manual Professional Edition, Vitamin B12 Deficiency: link
• MSD Manual Consumer Version, Vitamin B12 Deficiency: link
• EFSA, Summary of Tolerable Upper Intake Levels, 2024 version (vitamin B12: no defined adverse effects): link
• Gelbe Liste Pharmindex, Hydroxocobalamin: Anwendung, Wirkung, Nebenwirkungen (in German): link
• European Medicines Agency, Cyanokit (hydroxocobalamin), summary of product characteristics (chromaturia): link
• Swiss drug compendium, Vitarubin Depot, hydroxocobalamin 1 mg/ml (in French): link
• MSD Manual Professional Edition, Overview of Vitamins (recommended intakes): link