Side Effects of Vitamin B: Hives, Rash and Heat

Too much vitamin B can cause allergic reactions.
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B-complex vitamins play a key role in energy metabolism and brain health. However, they may cause adverse effects, especially in people with a vitamin B allergy. Common reactions may include a vitamin B rash, hives and urticaria. In the worst-case scenario, these nutrients can lead to anaphylaxis.


Dietary supplements and injections contain large doses of B vitamins, which may increase the risk of side effects and allergic reactions.

Why Are B Vitamins Important?

Heath organizations worldwide recommend the consumption of leafy greens, whole grains, nuts, seeds and eggs. These foods are rich in micronutrients, especially B-complex vitamins. Grain products, for example, boast large doses of folic acid, niacin, thiamin and riboflavin. The USDA reports that one cup of raw rye provides more than one-third of the daily value (DV) of riboflavin, 45 percent of the DV of niacin and nearly half of the DV of pantothenic acid.


These water-soluble vitamins regulate cell metabolism, among other functions. Your body needs them to break down glucose and other dietary nutrients. Riboflavin, for instance, is a component of two coenzymes that support energy production and lipid metabolism. Pyridoxine, or vitamin B6, helps your body synthesize amino acids and create neurotransmitters.

Another essential vitamin is B12. About 1.5 to 15 percent of Americans are deficient in this nutrient, as reported by the National Institutes of Health. Fatigue, constipation, anemia, depression and memory problems are all common side effects of low vitamin B12 levels in the bloodstream. This nutrient is found mostly in animal foods, so vegans and vegetarians are more likely to develop deficiencies.


Read more: Benefits and Side Effects of Vitamin B12

As mentioned, B-complex vitamins are soluble in water. The excess is eliminated in the urine. In rare cases, allergic reactions and hypervitaminosis, or vitamin toxicity, may occur. Too much vitamin B6, for example, may cause numbness, digestive distress, severe skin lesions and poor muscle control, according to the Mayo Clinic.

Symptoms of Vitamin B Allergy

Some B vitamins may cause allergic reactions, from itching and hives to blisters and swelling. A review published in the journal Allergology International in August 2017 states that thiamin is the most allergenic vitamin. This nutrient supports cell growth and development, keeps your brain healthy and aids in the conversion of glucose into energy.


The above review describes the case of a 50-year old woman who developed allergic reactions after receiving vitamin B shots. Her symptoms, which included facial redness, swelling and hives, occurred 30 minutes later. Skin tests have shown that she was allergic to vitamin B1, or thiamin.

The researchers point out that injectable thiamin is more likely to cause side effects than oral administration. In rare cases, those who are allergic to this B vitamin may experience severe allergic reactions, such as anaphylaxis. Other potential side effects include itching, wheezing and difficulty swallowing.


Vitamin B12 may cause allergic reactions too, according to a May 2018 case report featured in Cureus. A 55-year-old vegan woman received several B12 shots for seven consecutive weeks. Her symptoms flared during the second week and included breathing problems, stomach pain, abdominal distension, hives, itchy skin and swelling of the tongue.

She received treatment for these symptoms and started to take oral vitamin B12 the following week — without experiencing any side effects.

Injectable vitamin B12 may contain impurities and preservatives with allergenic potential. Therefore, it carries higher risks compared to the oral form. It's interesting to note that vitamin B allergy symptoms can occur after months or even years of treatment, according to the Allergology International review.

Biotin, another B vitamin, isn't safer, either. This nutrient is often added to hair growth supplements and topical solutions. It's also marketed for its ability to strengthen the nails and improve skin appearance. In rare cases, it may cause hives, mild skin rash, gastrointestinal symptoms and facial swelling, according to the Michigan Medicine.

Read more: 10 Facts You Need to Know About Food Allergies

Other Potential Side Effects

B-complex vitamins may cause adverse reactions when used in large doses. In general, B vitamin supplements are the culprit.

Niacin supplements, for example, are prescribed for high cholesterol. A typical dose is 1 to 2 grams per day — that's a lot more than the daily recommended intake (15 milligrams). Such doses may produce a sensation of heat all over the body, especially to the face. This side effect is known as a niacin flush.

Virtually every person who takes large doses of niacin may experience intense flushing, according to the British Columbia Drug and Poison Information Centre (BC DPIC). This symptom usually occurs about 15 to 30 minutes after taking niacin supplements. However, it's just a nuisance and doesn't pose any danger.

Immediate-release niacin supplements are more likely to cause flushing. If you experience this side effect, consider switching to a sustained-release formula. The experts at the BC DPIC also recommend increasing the dose gradually, taking niacin tablets with food or adding aspirin to the mix. These simple measures may help reduce your symptoms.

Read more: 43 Supplements Exposed: Which Ones to Consider, Which Ones to Avoid

A study published in the Journal of Clinical Lipidology in March–April 2013 suggests taking apple pectin to reduce the intensity of niacin-induced flushing. However, more research is needed to confirm its efficacy. Pectin is a dietary fiber that occurs naturally in apples and citrus fruits. It's also available in supplement form.

Other side effects of B-complex vitamins may include elevated blood pressure, blurry vision, changes in urine color and digestive discomfort. The higher the dosage, the greater the risk. For example, large doses of nicotinic acid, a supplemental form of niacin, may cause liver toxicity. Ingesting large amounts of vitamin B6 may cause damage to the brain and nervous system.

Vegetables, whole grains, meat, eggs and other foods rich in B vitamins are unlikely to affect your health. Compared to supplements and injections, they're significantly lower in vitamin B.

The USDA reports that oatmeal, for example, provides 15 percent of the DV of thiamin, 4 percent of the DV of folate and 3 percent of the DV of niacin per serving (one cup). Cooked broccoli boasts 42 percent of the DV of folate, 18 percent of the DV of vitamin B6 and 15 percent of the DV of thiamin per serving (one cup). Each serving of salmon (3 ounces) delivers 114 percent of the DV of vitamin B12, 46 percent of the DV of niacin and 32 percent of the DV of vitamin B6.

Your doctor is the only one who can tell whether or not you need dietary supplements. To stay safe, consult your doctor before popping pills, so you can get a formula that meets your individual needs and has minimal or no side effects.