Signs & Symptoms of High B12

Vitamin tablets in a hand
High blood levels of B12 are rare. (Image: bee32/iStock/Getty Images)

High blood levels of B12 are rare. If your levels are high, you might not know it unless you visit a doctor and have a B12 blood test. Although there are a few potential side effects from any dose of B12, it carries little risk of causing serious symptoms or side effects, even at very large doses. Still, taking too much of any supplement isn't always safe — or effective. Speak with your physician about how much B12 you need and whether your blood levels are too high.

High Levels vs. High Doses

As a water-soluble vitamin, any B12 your body can't use is usually excreted in urine. It's not stored in the body, and you must replenish your source of the nutrient each day through your diet and supplements. Therefore, blood levels depend on your diet and supplementation at any given time.

The B12 normal range for healthy adults is between 160 to 950 picograms per milliliter (pg/mL). Anything over 950 (pg/mL) is considered high. There are a few reasons why your blood levels may be elevated:

If you've just had a B12 injection, your blood levels could be temporarily high. But once the extra B12 has been excreted, your blood levels will go down again. You may have side effects from the injection, but they'll go away when the B12 leaves your system.

If your blood levels are high and it's not due to high-dose supplements, your body may have difficulty using B12, which can cause a buildup in the blood. In this case, you may actually have a B12 deficiency, even though you have high blood levels. Symptoms of a B12 deficiency include fatigue and weakness.

Finally, high blood levels of B12 can be a sign of more serious health conditions, including liver disease and blood cancer. If your doctor is alarmed by your B12 levels, he'll likely perform repeated tests at intervals to confirm your status.

High B12 Symptoms

Everyone is different, and you may or may not experience symptoms from high B12. Depending on the cause of your high B12, your symptoms may be related to an underlying condition. Still, high doses of B12 via supplements or injections may cause temporary symptoms. Dizziness, anxiety, headache and nausea are potential signs of elevated B12 levels, Mayo Clinic reports.

According to WebMD, potential but unlikely side effects of B12 injections include muscle cramps, weakness and irregular heartbeat. This is because the injection can cause blood levels of the mineral potassium to decrease. Therefore you may experience symptoms associated with potassium deficiency.

High doses of B12 may also affect your skin. According to a 2015 review in The American Journal of Clinical Dermatology, B12 supplementation may cause acne and rosacea. If you experience any side effects of any kind from taking B12 as a supplement or receiving an injection, talk to your doctor about whether you should continue taking the vitamin.

B12 Functions and Benefits

B12's primary function is the formation of healthy red blood cells. Your red blood cells contain a protein called hemoglobin, responsible for carrying oxygen throughout your body. In people with low B12 levels, weakness and fatigue are common symptoms. However, the opposite — high energy — is not a sign of high B12. Although many people take this supplement with the goal of improving their energy levels, endurance and sports performance, B12 increases energy only in those with a deficiency.

B12 is also in charge of controlling blood levels of an amino acid called homocysteine. A marker of inflammation, high levels of homocysteine are associated with heart disease, cognitive decline and depression. Although high B12 does decrease levels of homocysteine, that decrease has not been shown to reduce the risk of heart disease or cognitive decline. However, high levels of B12 may improve symptoms of depression in healthy adults, according to a 2013 study in ISRN Psychiatry.

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