Vitamin B12 injections vs. tablets

How B12 is normally absorbed

Vitamin B12 from food is bound to protein. Stomach acid releases it, and then it attaches to a protein called intrinsic factor, made in the stomach, which carries it to the end of the small intestine for absorption. Anything that disrupts those steps can lead to deficiency, even if you eat plenty of B12.

Some B12 can also be absorbed without intrinsic factor by passive diffusion, but only a small fraction. At high supplement doses, absorption is roughly 1.3% at 1,000 mcg, which is why oral treatment for deficiency uses large doses.

Who is more likely to run low

Midlife and older adults are more likely to have reduced stomach acid or atrophic gastritis, which makes food-bound B12 harder to release. Several common medications also play a role.

• Older adults, especially with atrophic gastritis
• People with pernicious anemia, an autoimmune condition that blocks intrinsic factor
• People with celiac disease, Crohn's disease or past stomach or bowel surgery, including gastric bypass
• Long-term metformin users
• Long-term users of acid-reducing medicines such as proton pump inhibitors or H2 blockers
• Vegans and some vegetarians

When injections are preferred

Injections are the traditional treatment for B12 deficiency because they bypass every barrier to absorption. They are often preferred when levels are very low, when there are neurological symptoms such as numbness, tingling, balance problems or memory changes, and in people with severe absorption problems. Nerve damage from long-standing deficiency can become permanent, so prompt, reliable treatment matters.

In pernicious anemia, treatment often starts with more frequent shots and then moves to a regular maintenance schedule. Your clinician sets the dose and frequency.

When tablets may be enough

High-dose oral B12 can work even for some people with absorption problems, thanks to passive diffusion. The Cochrane review found that oral doses of 1,000 to 2,000 mcg normalized blood levels about as well as injections over the short term, though it included only three small trials, so certainty is limited.

For people with mild deficiency, dietary causes or borderline levels and no neurological symptoms, tablets are often a reasonable, low-cost first step, provided levels are rechecked to confirm they're responding.

Forms, side effects and safety

B12 is generally very well tolerated by either route. Injections can cause soreness or redness at the site, and occasionally mild diarrhea or itching. Tablets rarely cause side effects. Common forms include cyanocobalamin and methylcobalamin; your clinician will explain which form is used in your treatment and whether it's an FDA-approved product or compounded.

It's important to find out why B12 is low, not just to replace it. If you take metformin, tell your clinician, since metformin can lower B12 absorption over time.

Monitoring and follow-up

Whichever route you use, follow-up testing tells you whether it's working. Clinicians usually recheck B12, and sometimes related markers such as a complete blood count, after a period of treatment. If you started on injections because levels were very low, your clinician may later discuss whether a maintenance plan with tablets is reasonable, or whether injections should continue.

Symptoms matter too. Report any new or worsening numbness, tingling, balance problems, memory changes or low mood, even if your blood levels look better, because nerve recovery can lag behind blood results. And don't stop treatment for a confirmed absorption problem without talking to your clinician, since levels can fall again over months.

Which might fit you

Your clinician will consider your lab results, symptoms and the likely cause of low B12. These general patterns may help you prepare.

• Diagnosed pernicious anemia, severe deficiency or nerve symptoms: injections are commonly used, at least to start
• Mild deficiency from diet or borderline levels: high-dose tablets are often tried first, with follow-up testing
• You dislike needles and your clinician agrees oral is appropriate: tablets with rechecked levels
• You've had stomach or bowel surgery: discuss injections, since absorption may be unreliable
• Your levels are normal: extra B12 is unlikely to boost energy, and fatigue deserves a broader look