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B12 Injection Dosage: What Is Right For You?

Sep 26, 2020
B12 Injection Dosage: What Is Right For You?

Have you been extra tired lately?  Have you noticed a lack of concentration, bouts of constipation, balance troubles, and/or memory loss?  If so, you might consider speaking with your doctor. There could be a possible connection between your lower energy levels and low levels of vitamin B12. Prompt attention to these symptoms could improve your quality of life.  If you are among the growing community of people who have been diagnosed with a vitamin B12 deficiency, you will want to discuss what form of vitamin B12 supplements would be best for you and in what amounts.  Doctors often prescribe a B12 injection dosage because of the ease of absorption.

Vitamin B12 – The Fundamentals

Vitamin B12, alternatively known as cyanocobalamin, is a member of the B vitamin family. The B vitamin family consists of eight essential nutrients. Each member of this family of vitamins performs specific and necessary tasks.  Vitamin B12 is needed for proper red blood cell formation and aids nerve function. It also plays a part in synthesizing DNA and converting fats into energy. Healthy amounts of vitamin B12 can prevent some cases of megaloblastic anemia, a certain type of anemia characterized by oversized, underdeveloped, and sparse red blood cells.  These abnormal cells lower the amount of oxygen that can be transported from the blood to the body’s tissues.

B12 Injection Dosage: What Is Right For You?

Vitamin B12 in its natural form is connected to protein in the foods that we eat. It can be found in poultry, meat, fish, eggs, milk, and certain fortified cereals and yeasts. Rainbow trout, salmon, and tuna are the next best sources of vitamin B, after beef liver and clams.  Absorption of the nutrient is a two-step process. 

  1. In the first step, acid manufactured in the stomach, called hydrochloric acid, detaches vitamin B12 from the protein.
  2. Second, a stomach-produced protein called intrinsic factor binds with vitamin B12, making it absorbable by the rest of the body.

The amount of vitamin B12 a person needs each day depends on their age:

  • Newborns start out requiring 0.4 micrograms (mcg) per day.
  • At 7 months into their first year of life, babies need 0.5 mcg. 
  • For the next three years of life, 0.9 mcg a day is needed. 
  • Children from the ages of four through eight need 1.2 mcg of vitamin B12. 
  • Older children aged 9–13 require 1.8 mcg.
  • Teenagers 14–18 and adults need 2.4 mcg daily. 
  • Pregnant teens and women need 2.6 mcg.
  • Breastfeeding teens and women require 2.8 mcg each day. (NIH, n.d.)

Symptoms Of Vitamin B12 Deficiency

Vitamin B12 deficiency can cause both physical and neurological symptoms.  Physically, low levels of vitamin B12 can cause megaloblastic anemia, fatigue, weakness, constipation, appetite loss, and weight loss.  Neurological indicators of low vitamin B12 levels include tingling or numbness in hands and/or feet, dizziness, balance problems, dementia, confusion, memory loss, mouth and tongue soreness, and depression.  These neurological issues can occur without anemia and can become permanent if not rectified promptly (NIH, n.d.)      

B12 Injection Dosage: What Is Right For You?

Reasons For Vitamin B12 Deficiency

With a wide range of natural sources, most people get enough vitamin B12 through their diet; however, some individuals are vitamin B12 deficient and may need supplemental vitamin B12. People in the following categories may not get sufficient amounts of vitamin B12 or may not be able to absorb it well:

  • Older adults, 50 and up, start to produce less of the hydrochloric acid needed to start the two-step absorption process.  Fortunately for this group, most can still absorb vitamin B12 from fortified sources.
  • Those who live with a condition called pernicious anemia lack the intrinsic factor needed in the second part of the nutrients two-step process and cannot absorb Vitamin B12. These individuals are usually prescribed vitamin B12 injections.
  • Gastrointestinal surgeries such as weight loss surgeries and surgeries where portions of the stomach are removed can decrease the amount of vitamin B12 available for the body to absorb.
  • People living with certain digestive disorders, like celiac disease or Crohn’s disease, also have decreased amounts of vitamin B12 that can be absorbed.
  • Vegetarians and vegans who do not eat any or eat insufficient amounts of animal proteins may also be vitamin B12 deficient.
  • Those who drink large amounts of alcoholic beverages.

People in these categories are typically the ones prescribed vitamin B12 injections.  For these individuals, the correct vitamin B12 injection dosage can greatly improve their health.  While high doses of oral vitamin B12 may be effective, doctors typically treat vitamin B12 deficiency with injections because this method bypasses potential barriers to absorption. The number one deciding factor between intramuscular injections and oral administration is the patient’s ability to absorb vitamin B12.  When determining the proper vitamin B12 injection dosage, decisions vary based on why the vitamin B12 injection is needed. 

Doctor discussing B12 Injection Dosage with a patient

Schilling Test, its phases, and its vitamin B12 injection dosage

  • Historically, once doctors established the presence of vitamin B12 deficiency, they often used a procedure called the Schilling Test to look for the cause of vitamin deficiency. This test is described for its historical significance, and it provides a way to see the major factors that influence B12 absorption. The Shilling test is rarely done nowadays because labeled cobalamine can no longer be obtained (Ramphul & Mejias, 2020).
  • Preparation for the Schilling test includes not receiving intramuscular injections of vitamin B12 for three days before the test and only drinking water for 8 hours before the test.  
  • Doctors typically give the test after a series of supplements have restored B12 to normal levels. The chart below outlines the four stages of the test.  It is important to note that each stage is dependent on an abnormal result from the previous stage.  If a normal result occurs, the test does not have to continue.

Schilling Test

Phase 1• During the first phase of the Schilling test, a patient receives two doses of vitamin B12. First, the patient gets an oral dose containing a dye that can be detected in the urine. (This kind of dye is called “radiolabeled” because it contains a harmless radioactive element that doctors use to trail a compound through the body.)
• After an hour, the patient receives a vitamin B12 injection. The typical vitamin B12 injection dosage is 1000 mcg. While this dose alone is insufficient to remedy a vitamin B12 deficiency, it is enough to observe the body’s absorption level.
• After receiving the vitamin B12 injection, the patient is sent home and instructed to return in 24 hours after collecting a 24-hour urine sample. The doctor will use the sample to test the rate of absorption and excretion.
Phase 2
(Performed within 3 to 7 days of an abnormal result from the patient’s initial urine sample.)
• To test the function of the pancreas and its role, if any, in the deficiency, the doctor will give the patient a three-day supply of pancreatic enzymes.
• Then the doctor will administer the final dose of oral vitamin B12 with the dye and send the patient home for 24 hours with instruction to collect a final sample to be analyzed.
Phase 3• During this phase, the doctor will prescribe a two-week round of antibiotics.
• The patient will then come and receive another oral dosage of vitamin B12 and order the collection of another urine sample in 24 hours.
• If this test comes back normal, bacterial overgrowth might have caused the deficiency. If it comes back abnormal, the doctor will move on to the final phase of the test.
Phase 4• To test the function of the pancreas and its role, if any, in the deficiency, the doctor will give the patient a three-day supply of pancreatic enzymes.
• Then the doctor will administer the final dose of oral vitamin B12 with the dye and send the patient home for 24 hours with instructions to collect a final sample to be analyzed.
Source: UCSF Benioff Children’s Hospital

Concurrent Medications May Affect Vitamin B12 Injection Dosage

While most medicines are not affected by supplemental cyanocobalamin, there are fifteen medications that cause some level of interaction with a person’s vitamin B12 dosage. 

  • aminosalicylic acid (non-specific mild reaction)
  • arsenic trioxide
    • Used to treat a type of cancer called acute promyelocytic leukemia, this medicine could cause an irregular heart rhythm that might become serious and potentially life-threatening.  Vitamin B12 injection dosage might increase the risk of this occurrence.  This is because as vitamin B12 replenishes, it can cause low blood potassium, which in turn can also cause irregular heart rhythm.  Anyone taking these medications together should talk with their doctor about their concerns. Using both could require dose adjustments and/or more frequent monitoring.)
  • chloramphenicol
    • This antibiotic (used to treat various bacterial infections such as meningitis, plague, cholera, and typhoid fever) could hinder bone marrow function, which would in turn interfere with the effectiveness of the vitamin B12 injection dosage. The doctor may want to watch these drug interactions a little more closely.  Do not hesitate to consult your doctor if symptoms get worse or conditions change.
  • cimetidine (non-specific mild reaction)
  • dexlansoprazole (non-specific mild reaction)
  • esomeprazole (non-specific mild reaction)
  • famotidine (non-specific mild reaction)
  • lansoprazole (non-specific mild reaction)
  • nizatidine (non-specific mild reaction)
  • omeprazole (non-specific mild reaction)
  • pantoprazole (non-specific mild reaction)
  • potassium chloride (non-specific mild reaction)
  • rabeprazole (non-specific mild reaction)
  • ranitidine (non-specific mild reaction)
  • ranitidine bismuth citrate (non-specific mild reaction) (, n.d.)

As this list indicates, it is important to be proactive about communicating any side effects to your doctor promptly. This list is not meant to be complete or to be used for self-diagnostic purposes.

Frequently Asked Questions

Do I have to go to the doctor for every vitamin B12 dosage?

Not necessarily. If you feel comfortable administering the injections yourself or having someone trusted to give you the shot, you can ask your doctor to show you how to administer the shot at home. Also, patients can buy b12 shots online.

Why can’t vitamin B12 be injected right into the veins?

Intravenous injections are not recommended because all of the vitamin B12 would be lost in the urine.

Are there any diet restrictions I should adhere to while taking vitamin B12?

When taking vitamin B12, it’s important to limit alcohol intake. Even moderate use of alcohol could decrease vitamin B12 levels.

Concluding Thoughts

Vitamin B12 is an essential nutrient, naturally found in animal proteins or fortified into cereals or yeasts, and most people can maintain proper levels through their diet. However, for those experiencing symptoms of low vitamin B12 levels, it’s essential to consult with your doctor or healthcare professional to determine the appropriate dosage. When discussing your prescription with your doctor, ensure they are aware of all your medications and symptoms.

To obtain your vitamin B12 injection dosage from a trusted source, consider Invigor Medical, a US-based pharmacy known for excellent compounding with a strong focus on customer health. When you buy Vitamin B12 through Invigor Medical, you can have confidence in the quality and reliability of your medication.

While we strive to always provide accurate, current, and safe advice in all of our articles and guides, it’s important to stress that they are no substitute for medical advice from a doctor or healthcare provider. You should always consult a practicing professional who can diagnose your specific case. The content we’ve included in this guide is merely meant to be informational and does not constitute medical advice.


  • National Heart, Lung and Blood Institute. (n.d.). Pernicious Anemia.
  • Ramphul K, Mejias SG. Schilling Test. [Updated 2020 Sep 15]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-.
  • UCSF Benioff Children’s Hospital. (n.d.) Shilling Test.
  • (n.d.) Vitamin B12 (cyanocobalamin) Drug Interactions.


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