Can you have any reactions or side effects from vitamin B12 injections? Most medical documents indicate you might experience any of these:
- Mild diarrhea and/or upset stomach
- Difficulty breathing or swallowing
- Weight loss or weight gain
- Nervousness or restlessness
- Nausea or vomiting
- Headaches or migraines
- Paresthesias (tingling sensations)
- Rhinitis (chronic sneezing or a congested, drippy nose)
- Skin reactions, such as rash, itching, swelling, blisters, or pain at the site of injection
- Light joint pain
- Lightheadedness or dizziness
- Chest pain or discomfort
- Low blood potassium levels
- Irregular heartbeat
- Chills, night sweats, fever, or hot flushes
- General feeling of sickness
However, people often fail to recognize that many side effects of vitamin B12 injections are consistent with B12 deficiencyitself. This overlap occurs because when patients have suffered a B12 deficiency through a disease like pernicious anemia, doctors often only diagnose the disease and start giving B12 shots at its end stage. But, adverse effects are recorded in a way that necessitates their inclusion as B12 injection symptoms.
So, No Side Effects to B12 Injections?
B12 is water-soluble, and it’s generally safe even at very high doses. In the complementary medicine realm B12 is probably one of the safest things you could take.
B12 is a water-soluble vitamin, meaning any excess is quickly flushed out through the urine. You can’t overdose, and there’s no toxicity threshold. It’s harmless and often used in ultra-high doses. For example, B12 shots of 25-50 mg doses (nearly a whole monthly vial of ours) have been used to treat cyanide poisoning, neuropathy, and ALS.
That being said, the conversion of severe anemia to normal production of red blood cells can temporarily lead to low potassium levels and a high platelet count. Therefore, monitor these two, and you may want to increase your potassium intake in the first few months of injecting. Read our pernicious anemia diet page for more advice.
Allergic Reactions to B12 Injections
Though rare, anaphylactic reactions (strong allergic responses) have been reported to either cyanocobalamin or hydroxocobalamin. Here are some cases:
- A 52-year-old woman reacted to an intramuscular cyanocobalamin B12 shot, but later received oral B12 with no adverse effects.
- Five years after starting her treatment, an 89-year-old woman developed signs of malaise, collapse, and vomiting an hour after a Neo-Cytamen shot (hydroxy B12).
- Another case involved an anaphylactic shock after a hydroxocobalamin injection.
- A 55-year-old woman suffered an allergic reaction after her second dose of an intramuscular cyanocobalamin B12 shot.
- A 54-year-old patient with pernicious anemia experienced anaphylactic shock twenty minutes after his sixth hydroxocobalamin shot.
- A 42-year-old woman developed an immediate reaction to intramuscular cyanocobalamin B12, despite positive intradermal testing.
- A rare case involved a woman who had immediate adverse reactions to both cyanocobalamin and hydroxocobalamin, by both injection and oral delivery.
- Cases of allergic reactions to hydroxocobalamin with tolerance to cyanocobalamin were documented in several instances (1, 2, 3).
Is The B12 Causing The Reactions?
Allergy to vitamin B12 is uncommon. However, once diagnosed it presents a difficult management problem in patients suffering from pernicious anemia, as there is no substitute.
Allergy to vitamin B12
The cases above shouldn’t scare you. We scoured the medical database for reports of allergic reactions to B12 shots, and this is all we could find. Keep in mind that millions of people around the world have received B12 shots. Considering the number of patients and the few reported cases, you can see how rare these reactions really are.
What About Simply Feeling Worse Off?
If you ever feel worse after a B12 injection, the cause is most likely not the B12 itself, but some troublesome excipient. For example, additives such as carboxymethyl cellulose and mannitol can cause acute reactions. It could also be the base fluid. If anything other than sterile saline was added, it may have been a preservative that you had reacted to.
Benzyl alcohol is a common issue. Here’s a case of a 67-year-old man who reacted to a hydroxocobalamin B12 shot containing 1.5% benzyl alcohol. Here’s a verified case of adverse reactions to vitamin B12 shots due to benzyl alcohol sensitivity.
Allergic reactions to proprietary preparations of vitamin B12 might be due to substances added to the solutions as preservatives or for other reasons, to impurities from the production, or to the vitamin B12 itself.
Anaphylactic reaction after injection of vitamin B12
Here’s a case of two female patients with pernicious anemia who had adverse reactions to B12 shots (hydroxocobalamin). The author of the study, Dr. Rosa Caballero, stated that the allergens involved in vitamin B12 reactions is likely to be a hapten, a small molecule that can trigger an immune response when it binds to a larger carrier protein.
Lastly, there’s Dr. Chandy’s hypothesis that a sudden influx of B12 can trigger repair processes. If there has been long-standing damage, this triggering might overwhelm the system, temporarily worsening symptoms. We discuss this in detail here.
Does Our B12 Have Any Side Effects?
Many readers of this website self-inject regularly. However, we have never had anyone come back to us with an allergic reaction to our B12. Not a single case. We have many repeat customers, so it must tell you something about how rare a true allergy is.
Our injectable B12 is methylcobalamin, which is an active, native form with no recorded anaphylactic reactions in medical literature. Zero cases. It comes 100% pure, without additives or preservatives. Methyl B12 is the most absorbable form and has distinct nerve-repairing properties, and for most people it is the best form.
Important: Patients with Leber hereditary optic neuropathy should not take methyl B12 shots. Hydroxy B12 shots, though less effective, are safer, as this disease can get worse in those with high methylcobalamin levels.
If you order your B12 shots elsewhere, make sure they come from a certified lab that follows stringent pharmacy regulations. Also, check that they don’t contain benzyl alcohol, or at least verify that you’re not sensitive to it. If you’re injecting a premature newborn, avoid benzyl alcohol entirely due to the risk of gasping syndrome.
Finally, avoid cyanocobalamin. It is a synthetic form of B12 with cyanide attached to the cobalamin (the B12). Many patients with B12 deficiency, especially those with pernicious anemia, have difficulty clearing cyanide. Cyano B12 can make their health worse.
What If I’m Still Worried?
If you suspect a cobalt allergy (which is extremely rare), have your doctor supervise an intradermal test dose first. Look for reactions such as a skin rash, swelling, a blue tint to your skin, or chest tightness. Consider having your doctor perform the test with our pure methyl B12, which again, nobody has ever reported any negative reaction to.
To Summarize
Vitamin B12 injections side effects are incredibly rare, and when reactions occur, they are more likely due to additives rather than the B12 itself. If you want to make sure your B12 is free of additives, you can buy a vial of our pure methylcobalamin B12.
Good luck.
