After years of symptoms and possibly decades of misdiagnosis, starting B12 injections should bring relief. Sometimes, though, people feel worse instead of better. For many people, it feels like “two steps forward, one step back.”
Possible Explanations
About 10% of people notice their symptoms intensify in the first few weeks after starting B12 treatment. For some people, symptoms like tingling, burning, or shooting nerve pain may increase. Others notice brain fog, agitation, or mood changes. Several explanations for this pattern have been proposed. None has been confirmed by direct research.
Clinical Hypotheses
Dr. Joseph Chandy, a UK general practitioner who treated B12 deficiency for over 45 years, observed this pattern in an estimated one in ten of his patients. His explanation was that B12 allows the body to begin correcting long-standing damage all at once, and when that damage has accumulated for years, the concentrated repair work can temporarily worsen symptoms before they improve.
Other explanations have also been proposed. In patients with severe anemia, rapid new blood cell production in the first days of treatment can pull potassium into cells faster than the body replaces it, sometimes causing a significant drop in blood potassium. Unrelated coexisting conditions, such as vitamin D deficiency, may worsen independently around the same time. And in some cases, symptoms that appear to worsen after starting treatment may simply reflect a B12 deficiency that hasn’t yet been adequately corrected, particularly with inadequate dosing. Neurological symptoms in particular require a dosing frequency that many doctors do not follow.
Nerve pathways recovering from prolonged B12 deficiency may also become temporarily more active or sensitive as they resume normal signaling. This could account for the range of sensory, cognitive, and mood-related symptoms people report.
Possible Repair Pathway
Myelin damage → repair begins → nerve pathways become more active → temporary increase in sensitivity → symptom intensification
Biology of Remyelination
B12 deficiency damages myelin, the coating that protects nerve fibers and helps them send signals properly. Once B12 treatment starts, the body begins rebuilding that coating, a process called remyelination.
This repair explains why B12 treatment can reverse nerve damage caused by deficiency. We don’t know if this repair process causes the temporary symptom increase, and it may be difficult to study directly. Newly diagnosed PA patients need immediate treatment. It would be unethical to delay for a clinical trial.
What to Expect
In the first 1-2 weeks: some people notice symptoms get worse before they get better. This is often when repair processes are most active.
Weeks 2-6: Most people begin to see gradual improvement, though symptoms may still fluctuate up and down.
6 weeks to 3 months: The majority of improvement typically occurs during this period, though some people continue improving for much longer.
The timeline varies greatly between individuals. Some people notice improvement within days, while others need months. How long B12 deficiency existed before treatment, how severe the damage became, and individual response patterns all affect recovery.
Is This Normal or Concerning?
This is typically normal if symptoms worsen temporarily (usually 1-3 weeks), the worsening feels similar to original symptoms just more intense, and treatment continues consistently.
Contact a doctor if:
- Symptoms continue getting worse after 3 months of treatment
- Completely new symptoms develop that never existed before
- Symptoms improve then suddenly get much worse again
- Concerns arise about the treatment plan
Managing the Initial Period
Don’t stop injections. Temporary worsening is part of the healing process, not a sign that treatment isn’t working.
Track symptoms. Keep a simple record of daily symptoms to identify patterns and progress over time.
Be patient with the process. Nerve healing takes time. Most people see their best improvement in the first few months, but some continue improving for a year or longer.
Communicate with healthcare providers. If symptoms seem severe or concerning, discuss whether injection frequency needs adjustment. Some people need more frequent doses during initial healing.
Support overall health. Get adequate sleep, eat nutritiously, and avoid alcohol, which interferes with B12 absorption and nerve healing.
Closing
No one knows for certain why this temporary worsening happens. What is clear is that it isn’t uncommon, and for most people, it resolves as treatment continues.
If symptoms continue to worsen after 3 months, it may point to a cellular issue in addition to, or instead of, pernicious anemia. See B12 Beyond the Basics: Cellular Resistance and Functional Deficiencies: https://dittany.com/b12-beyond-the-basics-cellular-resistance/.
For more on pernicious anemia and B12 deficiency, see related guides: https://dittany.com/pa-c/.
For Further Information
Dr Joseph “Chandy” Kayyalackom — A Reflection, Pernicious Anaemia Society.
https://pernicious-anaemia-society.org/pernicious-anaemia/dr-joseph-chandy-kayyalackom-a-reflection/
Background on Dr. Chandy’s clinical career and the observations behind his hypothesis on temporary worsening during treatment.
Hypokalemic paralysis in an adolescent following dexamethasone and B12 injection: A case report and literature review, PMC.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11782966/
Documents B12-induced hypokalemia in pernicious anemia patients.
Megaloblastic Anemia Treatment & Management, Medscape.
https://emedicine.medscape.com/article/204066-treatment
Clinical reference confirming that serum potassium can fall during treatment for severe cobalamin deficiency and should be monitored, with supplementation given as needed.
Lhermitte’s Phenomenon as the Presenting Symptom of Vitamin B12 Deficiency With Complete Reversibility of Neurological and Radiological Abnormalities, PMC.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12672955/
A case report documenting MRI-confirmed remyelination and full symptom resolution following B12 treatment.
Obeid R, Andrès E, Češka R, et al. Diagnosis, treatment and long-term management of vitamin B12 deficiency in adults: a Delphi expert consensus. J Clin Med. 2024;13(8):2176. https://doi.org/10.3390/jcm13082176