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How Fast Will B-12 Work?

author image Katie Strzeszewski
Katie Strzeszewski has been writing professionally since 2003 and holds a Bachelor of Arts in professional writing and English secondary education from York College of Pennsylvania. Strzeszewski spent two years performing computer repair for Geek Squad, currently works for men's clothier Paul Fredrick and is also a competitive West Coast Swing and Hustle dancer.
How Fast Will B-12 Work?
How quickly B-12 works varies based on your current B-12 levels.

B-12 is a water-soluble vitamin that appears naturally in many animal-based foods. Vitamin B-12 controls your body’s ability to produce red blood cells and synthesize DNA, and it also impacts your neurological functioning. You don’t need much vitamin B-12 for your body to function properly -- the recommended dietary allowance for an adult is 2.4 micrograms. If you are vitamin B-12 deficient, you will need significantly more than the RDA to even out your levels of B-12. How quickly B-12 works depends on several factors including your level of B-12 deficiency and what type of supplement you take.

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Injections are the most popular way to treat vitamin B-12 deficiency. The B-12, usually in doses of 1,000 micrograms, is injected directly into your muscles, eliminating any possible absorption barriers. Treatment schedules can vary, but most injections are done over a three-month period on days one, three, seven, 10, 14, 21, 30, 60 and 90. For some people, the three-month period will suffice, but people who are severely B-12 deficient will have to continue with their B-12 shots on a monthly basis.

B-12 Supplements

People who are squeamish about injections can opt for oral B-12 supplements. An oral B-12 supplements treatment often starts with higher dosages, such as 2,000 micrograms daily, which is later reduced to 1,000 micrograms daily. Depending on your individual case of B-12 deficiency, your doctor may then lower your recommended dose to 1,000 mcg per week or 1,000 micrograms per month. Depending on your reaction to the oral B-12 supplements, your doctor may eventually remove you from B-12 completely. As with the injections, a timeline for reduced dosage varies based on your reaction to the B-12 supplement.


If you are mildly B-12 deficient, your doctor may recommend using a multivitamin instead of a B-12 supplement. A multivitamin will provide you with a lower dose of B-12 that will still be strong enough to level out your vitamin B-12 requirements.


There is no need to worry about overdosing on vitamin B-12, as excessive B-12 does not cause any adverse physical effects. There are medications, however, that can impede your body’s ability to process a B-12 supplement. Metformin, often prescribed for diabetes, and chloramphenicol, a bacteriostatic antibiotic, will affect your ability to absorb vitamin B-12. Other medications such as omeprazole, lansoprazole, cimetidine, famotidine and ranitidine, all of which treat peptic ulcer disease, will also interfere with your levels of B-12. Consult your doctor for information on how to raise or maintain your levels of vitamin B-12 if you are using these medications.

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