Does B12 Injection Cause Fever? Separating Fact from Concern

October 10, 2025 •

4 min read

While most B12 injections are administered without incident, a low-grade fever is a possible, though less common, side effect following the injection. This reaction is typically mild and temporary, representing one of several factors that could cause a slight rise in body temperature after a vitamin B12 shot.

Quick Summary

A B12 injection can cause a mild, temporary fever as a side effect or due to an immune response. Rarely, a high fever can signal a more serious allergic reaction or injection site infection. Underlying B12 deficiency may also present with fever that resolves with treatment.

Key Points

Mild Fever is Possible: A low-grade fever or chills are listed as less common, temporary side effects of B12 injections.
Immune Response: The fever is often caused by the body's mild immune response to the injected compound and typically resolves quickly.
Pre-Existing Deficiency: Fever can be a symptom of severe B12 deficiency (megaloblastic anemia) that resolves during treatment.
Allergic Reaction: A high fever can signal a rare but serious allergic reaction, which may also include hives, swelling, and breathing difficulties.
Injection Site Infection: A slight risk of infection exists with any injection, and a fever with increased pain or redness is a warning sign.
When to Seek Help: While mild reactions are manageable, seek immediate medical attention for a high fever or signs of a severe allergic reaction, like trouble breathing or facial swelling.

What is a B12 Injection?

Vitamin B12 injections, typically containing cyanocobalamin or hydroxocobalamin, are prescribed to treat or prevent vitamin B12 deficiency. This deficiency can result from conditions like pernicious anemia, certain surgical procedures, or dietary restrictions. Unlike oral supplements, injections bypass the digestive system, allowing for direct absorption into the bloodstream, which is essential for individuals with absorption issues.

Does a B12 Injection Cause Fever?

Yes, a B12 injection can cause a fever, but it is not a common side effect and is typically mild. Multiple medical sources, including patient information leaflets and clinical reviews, list fever and chills as less serious potential side effects of intramuscular cyanocobalamin or hydroxocobalamin. It's crucial to understand the different reasons why a fever might occur after a B12 injection.

Potential Causes of Fever After a B12 Injection

Several factors can lead to a fever following a B12 injection, ranging from a normal physiological response to a more serious medical issue.

Mild Immune System Response: The body's immune system can sometimes react to the injection of a new compound, leading to a temporary, low-grade fever. This is similar to the body's reaction to other types of injections and usually subsides on its own within 24 hours.
Underlying B12 Deficiency: In some cases, a fever that was a symptom of a severe, pre-existing B12 deficiency (often linked to megaloblastic anemia) may resolve or fluctuate as treatment begins. This paradox can sometimes be misconstrued as a side effect of the injection itself, when it is actually a symptom of the underlying condition improving.
Allergic Reaction: While rare, a more significant fever can be part of a systemic allergic reaction to the injection. This can be an allergy to the active ingredient (cobalamin) or, more commonly, to a preservative or inactive ingredient in the solution. Allergic reactions can be immediate or delayed and may also include symptoms like hives, itching, or swelling.
Injection Site Infection: As with any injection, there is a small risk of infection at the injection site. A fever can be a sign that an infection has developed. Other symptoms would include increasing pain, redness, warmth, or pus at the injection area.

Differentiating Between Mild Side Effects and Serious Reactions

It is important for patients to be able to distinguish between a minor, temporary reaction and a more serious medical event. The table below outlines key differences.


Symptom	Mild Reaction (Less Serious)	Serious Reaction (Seek Medical Attention)
Fever	Low-grade (under 101°F), subsides within a day or two.	High-grade (over 101°F) or accompanied by other severe symptoms.
Injection Site	Mild pain, redness, or swelling that improves.	Severe, spreading pain, redness, warmth, or pus.
Systemic Symptoms	Mild headache, nausea, or feeling unwell.	Shortness of breath, chest pain, irregular heartbeat, severe dizziness, facial swelling.
Onset	Typically within hours of the injection.	Can be immediate (anaphylaxis) or delayed (delayed hypersensitivity).

What to Do If You Develop a Fever After a B12 Injection

For a Mild, Low-Grade Fever:
- Monitor: Keep track of your temperature. A low-grade fever that resolves within a day or two is often not a cause for alarm.
- Rest and Hydrate: Drink plenty of fluids to stay hydrated.
- Pain Relief: Consider over-the-counter pain relievers like acetaminophen or ibuprofen for discomfort, if appropriate for your health condition, and approved by your doctor.
For Serious or Worsening Symptoms:
- Contact Your Doctor: If the fever is high, persistent, or accompanied by other concerning symptoms, you should contact your healthcare provider for guidance.

When to Seek Immediate Medical Attention

While most post-injection symptoms are not severe, certain signs warrant immediate medical help. You should seek emergency care if you experience any of the following symptoms, as they can indicate a severe allergic reaction (anaphylaxis):

Difficulty breathing or wheezing
Swelling of the face, tongue, or throat
Chest tightness or chest pain
Rapid or irregular heartbeat
Severe rash, widespread hives, or severe itching
Dizziness or lightheadedness

Conclusion

While a low-grade fever can occur after a B12 injection, it is typically a mild, temporary immune response and not a cause for concern. It is important to remember that fever could also be a symptom of the underlying deficiency itself, resolving with treatment. However, any persistent or high fever, especially when combined with other severe symptoms like swelling or difficulty breathing, requires immediate medical evaluation to rule out a serious allergic reaction or infection. Always inform your healthcare provider of any and all side effects you experience from medication to ensure your treatment is safe and effective.

For more detailed information on specific B12 preparations and their potential side effects, you can consult with your doctor or reference authoritative drug information databases.

Frequently Asked Questions

Fever is not a common side effect of B12 injections, but it can occur. When it does, it is typically a low-grade, temporary fever that resolves within a day or two.

Most often, if a fever occurs, it is a low-grade fever that is part of a mild immune response. High fevers are rare and could indicate a more serious underlying issue or allergic reaction.

Yes, some cases of severe B12 deficiency, particularly with megaloblastic anemia, can present with fever. Starting treatment may cause fluctuations in symptoms as the body recovers.

You should be concerned if you develop a high fever or experience other severe symptoms such as difficulty breathing, a spreading rash, facial swelling, or chest pain. These could be signs of a serious allergic reaction and require immediate medical attention.

Mild side effect fevers are low-grade and often subside within 24-48 hours. Fevers from allergic reactions are more likely to be higher and accompanied by symptoms like hives, itching, significant swelling, or difficulty breathing.

For a mild, low-grade fever, it is best to rest and stay hydrated. You can also take over-the-counter fever reducers if your doctor approves. Monitoring your symptoms is key to ensuring they do not worsen.

Both cyanocobalamin and hydroxocobalamin injections list fever as a possible, though less common, side effect. Hypersensitivity reactions are typically to inactive ingredients or preservatives rather than the cobalamin itself, but an allergy to the active ingredient is possible.