Should I take B12 when sick? A guide to vitamins and immune support

November 2, 2025 •

4 min read

Vitamin B12 is essential for red blood cell formation and nervous system function, and a deficiency can cause fatigue and weaken your immune system. Should I take B12 when sick? The answer depends heavily on your underlying health status and whether you have an existing deficiency or absorption issue.

Quick Summary

Supplemental B12 primarily helps those with a diagnosed deficiency, which can impair immune response. For general illnesses, focus on hydration and rest; consult a doctor before starting new supplements.

Key Points

Check for Deficiency: Supplementation is most beneficial for those with a pre-existing B12 deficiency, which can cause fatigue and weaken the immune system.
Rest and Hydrate First: For typical illnesses like a cold or flu, focusing on adequate rest and hydration is more effective than adding a B12 supplement.
B12 Boosts Immune Cells: Vitamin B12 aids in the production of immune cells and DNA synthesis, processes crucial for fighting infection.
Injections for Poor Absorption: Individuals with absorption disorders like pernicious anemia may benefit most from B12 injections, which bypass the digestive system entirely.
Dietary Intake is Key: For long-term immune support, ensure a diet rich in B12 from sources like meat, fish, eggs, and fortified foods.
Consult a Professional: Always talk to a doctor before starting new supplements, particularly if you have health conditions or take other medications.

The role of Vitamin B12 in immune function

Vitamin B12, or cobalamin, is a powerhouse nutrient involved in several critical bodily functions. Its contribution to a robust immune system is significant, particularly its role in creating red blood cells and synthesizing DNA. Immune cells rely on oxygen delivered by red blood cells to function effectively, making proper red blood cell production vital for fighting off pathogens. A deficiency in B12 can disrupt this process, potentially weakening your body's defenses against infections.

How B12 supports your body during illness

When you get sick, your body's cellular activity increases as it works to fight the infection. B12 is a cofactor for enzymes involved in DNA synthesis and energy production, which are essential for the rapid multiplication of immune cells. Maintaining adequate levels is therefore crucial for supporting a timely and effective immune response. While B12 is important, it is not a magic cure for illness. For otherwise healthy individuals, the body's stored B12, often sufficient for several years, is typically enough to get through a minor illness.

Deficiency vs. Supplementation During Sickness

For those with a confirmed B12 deficiency, supplementation is necessary and can aid recovery from illness. Common symptoms of deficiency include fatigue, weakness, and neurological issues, which can overlap with general sickness symptoms. For these individuals, correcting the deficiency is a priority. However, taking extra B12 when your levels are already sufficient has not been shown to provide additional benefits in fighting off a common cold or flu.

Factors affecting B12 levels and absorption

Dietary Habits: Vegans and vegetarians are at higher risk of B12 deficiency, as the vitamin is primarily found in animal products.
Age: Older adults often have reduced stomach acid, which impairs B12 absorption.
Medical Conditions: Conditions like Crohn's disease, celiac disease, and pernicious anemia can interfere with absorption.
Medications: Certain medications, such as metformin and proton pump inhibitors, can reduce B12 absorption over time.

Comparing B12 Delivery Methods for Illness

When considering B12, it is useful to understand the different ways it can be administered and what to expect from each. The following table compares oral supplements and injections.


Feature	Oral B12 Supplements	B12 Injections
Best for...	Treating diet-related deficiencies or maintaining levels in otherwise healthy individuals.	Treating moderate to severe deficiencies and conditions that impair absorption, like pernicious anemia.
Absorption	Can be low, especially with absorption issues, though high doses can overcome this through passive absorption.	Direct entry into the bloodstream results in higher, more immediate absorption rates, bypassing digestive issues.
Speed of Effect	Gradual improvement over weeks or months, depending on the dose and deficiency level.	Often provides a noticeable energy lift within 24-48 hours for those with a significant deficiency.
Availability	Available over-the-counter without a prescription.	Requires a doctor's prescription and administration, though some clinics offer them.
Cost	Generally low-cost and widely available.	Can be more expensive and may not be covered by insurance for non-deficiency cases.

What to do if you are sick

Unless you have a pre-existing B12 deficiency or a condition affecting absorption, adding a B12 supplement to your routine when you get sick is unlikely to make a major difference in the length or severity of your illness. Instead, focus on the basics of recovery:

Stay Hydrated: Drink plenty of water, clear broth, and herbal teas. Dehydration can worsen symptoms like fatigue and headaches.
Rest: Your body needs energy to fight infection. Adequate rest allows your immune system to work efficiently.
Balanced Diet: Consume nourishing foods rich in vitamins and minerals to support your body's healing process. Try to eat fruits, vegetables, and lean proteins.
Listen to Your Doctor: If you are feeling unwell for an extended period, or if you suspect a deficiency, consult a healthcare professional. They can perform tests and determine if supplementation is appropriate for you.

Food sources of B12

Instead of relying solely on supplements, ensuring a B12-rich diet is a proactive measure for long-term immune health. Good dietary sources include:

Meat, especially beef liver and clams
Fish, such as salmon and trout
Dairy products, like milk and cheese
Eggs
Fortified foods, including some breakfast cereals, plant-based milks, and nutritional yeast

Conclusion

For most people experiencing a common cold or flu, the question of whether they should take B12 when sick is secondary to foundational wellness practices. While B12 is a vital nutrient for immune health, a healthy individual's stores typically suffice during a temporary illness. Supplemental B12 offers the most significant benefits to those with a confirmed deficiency or absorption issues, helping to correct the root cause of fatigue and immune weakness. Focusing on rest, hydration, and a balanced diet remains the most effective strategy for managing general sickness. Always consult with a healthcare provider before beginning any new supplement regimen, especially if you have underlying health concerns or are on medication. The Office of Dietary Supplements at the National Institutes of Health offers comprehensive resources on vitamin B12 and other essential nutrients, emphasizing diet over supplementation for those without deficiencies.

Frequently Asked Questions

If you are not deficient in B12, there is no strong evidence that B12 shots will help you recover faster from a common cold. Their primary benefit is correcting a deficiency, which can improve symptoms like fatigue caused by low B12.

Symptoms of B12 deficiency can include fatigue, weakness, pale skin, a sore tongue, headaches, and neurological issues like tingling in the hands and feet or memory problems.

Because B12 is a water-soluble vitamin, excess amounts are typically flushed out in your urine. It is not considered toxic at high doses, though some people may experience mild side effects like nausea or diarrhea.

At-risk groups include vegans and vegetarians, older adults, people with digestive issues like Crohn's or celiac disease, and those on certain medications like metformin.

If you are not deficient, the standard advice is to focus on rest and hydration during an illness. For fatigue persisting after an illness, particularly if you are in a risk group, it may be worth getting your B12 levels checked.

Yes, but you must be diligent. Vegans and vegetarians should consume fortified foods such as cereals, nutritional yeast, and plant-based milks, or take a supplement, as B12 is not naturally present in plant foods.

B12 injections are administered directly into the muscle, leading to much higher absorption rates and quicker effects, making them ideal for severe deficiencies or absorption problems. Oral supplements rely on the digestive system and are better for mild deficiencies or maintenance.