What Could Possibly Go Wrong If Your Vitamin B12 Is Low?

Most people think vitamin B12 deficiency is pretty straightforward.

You feel tired. Maybe a little weak. Maybe a little foggy. You take a supplement, improve your diet, and move on.

That was my understanding too… UNTIL I started looking into the research.

The deeper I dug, the more I realized that vitamin B12 isn’t just an “energy vitamin.” It’s involved in producing healthy red blood cells, maintaining the nervous system, supporting brain function, and helping cells throughout the body work properly. When levels fall low enough for long enough, the consequences can show up in places most people would never connect to a vitamin deficiency.

Researchers have documented cases involving unexplained fevers, vision loss, psychiatric symptoms, memory problems, infertility, and even conditions that initially looked like dementia.

[To be clear, not everyone with low B12 will experience these outcomes. Many occur in severe or prolonged deficiency.]

PHYSICAL EFFECTS

The Unexplained Fevers

One of the first findings that genuinely surprised me involved fever.

Most of us associate fever with infection. If someone develops a temperature of 102°F, the obvious assumption is that they’re fighting a virus, bacteria, or some other illness.

Yet a study involving 34 patients with severe vitamin B12 deficiency found something unusual. These patients had persistent fevers despite investigations failing to identify an infectious cause. The common factor turned out to be severe megaloblastic anemia, a condition caused by impaired red blood cell production due to B12 deficiency.

Researchers believe the body’s ineffective attempt to produce healthy blood cells may trigger inflammatory responses that raise body temperature. What made the finding particularly striking was what happened after treatment. Once vitamin B12 therapy was started, the fevers typically disappeared within two to three days.

This doesn’t mean every unexplained fever is caused by low B12. Far from it. But it does show how deficiency can sometimes present in ways that seem completely unrelated to nutrition.

Tingling 

Many people with vitamin B12 deficiency describe tingling hands, numb feet, burning sensations, or a feeling that they’re walking on cotton.

At first glance, these symptoms don’t sound especially alarming. Most of us have experienced temporary pins and needles after sitting awkwardly.

The difference is that B12-related symptoms originate from nerve dysfunction.

Vitamin B12 plays an essential role in maintaining myelin, the protective coating that surrounds nerves. Think of myelin as insulation around electrical wires. Without adequate insulation, electrical signals become less reliable. The same thing happens inside the nervous system.

In some people, tingling is one of the earliest warning signs of deficiency. In others, numbness and altered sensation appear even before obvious anemia develops.

What makes this particularly important is that long-standing nerve damage may not always be fully reversible. Many patients improve dramatically with treatment, but neurological recovery becomes less predictable the longer deficiency persists.

Vision Loss 

Vision loss was another finding I wasn’t expecting.

Most people naturally assume worsening eyesight originates in the eyes themselves. However, several case reports have shown that severe vitamin B12 deficiency can damage the optic nerve.

One widely cited case involved a 63-year-old man whose vision deteriorated to approximately 20/400 in both eyes, a level considered legally blind in many places. Doctors eventually identified severe vitamin B12 deficiency as the underlying problem. After several months of treatment, his vision improved significantly, eventually allowing him to regain enough function to drive.

Not every patient experiences such recovery. But the case highlights how unexpectedly far-reaching the effects of deficiency can be.

Effects on Spinal Cord

This was the point where the research became genuinely difficult to ignore.

Neurologists have long recognized a condition called Subacute Combined Degeneration (SACD), in which severe vitamin B12 deficiency damages specific regions of the spinal cord.

Patients often begin with symptoms that seem relatively harmless: tingling feet, mild numbness, or occasional balance problems. Over time, however, these symptoms can progress to difficulty walking, impaired coordination, weakness, and loss of sensation.

What’s particularly striking is that the damage can often be seen on MRI scans. Radiologists frequently describe a characteristic “inverted V” pattern in the spinal cord, reflecting damage to nerve pathways that rely on healthy myelin.

MENTAL & PSYCHOLOGICAL FACTORS:

Deficiency can look Like Dementia

The cognitive effects can sometimes become much more severe.

One remarkable case report described a 52-year-old man who experienced rapid cognitive decline, severe enough that he stopped working and lost the ability to function independently. Brain imaging revealed extensive abnormalities in the white matter, which is the network of connections that allows different parts of the brain to communicate efficiently.

Doctors eventually discovered severe vitamin B12 deficiency.

Following long-term treatment, many of the brain abnormalities regressed and his cognitive function improved dramatically.

No one should conclude that most dementia is caused by B12 deficiency. It isn’t. But these cases highlight an important point: symptoms that appear to indicate permanent decline can sometimes have a reversible underlying cause.

Development of Psychosis

Perhaps the most surprising finding involved psychosis.

A 2024 case series described 8 patients admitted with acute psychosis associated with severe vitamin B12 deficiency. These individuals experienced hallucinations, delusions, confusion, and disturbances in thinking serious enough to require psychiatric care.

What made the cases particularly striking was that many lacked the classic signs people associate with B12 deficiency. Some had little or no obvious anemia. The psychiatric symptoms appeared to be the most visible problem.

After vitamin B12 treatment, alongside appropriate psychiatric management, substantial improvement was observed.

These cases don’t mean vitamin B12 deficiency is a common cause of psychosis. It isn’t. But they demonstrate just how dependent the brain is on proper nutrition.

SOCIAL AND RELATIONSHIP EFFECTS

People with vitamin B12 deficiency have reported becoming unusually irritable, emotionally reactive, anxious, or withdrawn. Family members are sometimes the first to notice that something feels different.

In patient accounts, individuals frequently describe feeling unlike themselves. Some report arguing more often, losing interest in social activities, or struggling to regulate emotions that once felt manageable.

These experiences are difficult to quantify in research studies, but they appear consistently across patient narratives. Health problems rarely affect only the individual. They often ripple outward into friendships, family dynamics, and romantic relationships.

Everyday Life Impacts

Beyond major medical complications, low B12 can quietly affect everyday life in ways that are easy to overlook.

One example involves nitrous oxide, commonly known as laughing gas. Researchers have documented cases in which nitrous oxide (Laughing Gas) exposure triggered neurological deterioration in people with underlying vitamin B12 deficiency. The gas can inactivate vitamin B12, effectively pushing already vulnerable individuals into symptomatic deficiency.

Researchers have also explored links between vitamin B12 status and fertility. Several studies suggest that deficiency may contribute to reproductive difficulties in both men and women, although fertility is influenced by many factors and B12 is only one piece of a much larger picture.

Even something as simple as being “clumsy” can occasionally have an unexpected explanation. Balance problems caused by nerve damage are sometimes mistaken for aging, poor coordination, or simple bad luck.

What I Took Away From All This

The biggest lesson I learned is that vitamin B12 isn’t just an energy vitamin. It’s a foundation vitamin.

Most people with low B12 won’t experience the severe complications described here. Risk depends heavily on how low levels are, how long they’ve been low, and individual circumstances.

But the research makes one thing clear: persistent symptoms deserve attention.

Fatigue, brain fog, numbness, balance problems, unexplained mood changes, or unusual neurological symptoms aren’t always caused by vitamin B12 deficiency. Yet sometimes they are.

And perhaps that’s the most surprising thing I found.

What looks like stress, aging, burnout, or a mysterious health problem may occasionally have a much simpler explanation: A nutrient the body has been missing all along.

References 

1. Sahoo, M., Dyuthy, R., & Sahu, A. (2024). Acute Psychosis as Presenting Symptoms of Vitamin B12 Deficiency: A Case Series. Cureus, 16(11), e74844.

2. Shiddappa, G., Mantri, N., Antin, S. S., & Dhananjaya. (2014). Megaloblastic Anemia Secondary to Vitamin B12 and Folate Deficiency Presenting As Acute Febrile Illness and Pyrexia of Unknown Origin: A Prospective Study from Tertiary Care Hospital. Scholars Journal of Applied Medical Sciences (SJAMS), 2(1D), 422-425.

3. Scharnweber, A. R., & Zimbalist, R. J. (2017). Vitamin B12 Deficiency Optic Neuropathy. The Journal of Optometric Education, 42(3).

4. Kapoor, N., Parihar, J., Dash, D., Tripathi, M., Bhatia, R., Garg, A., & Singh, R. K. (2021). Vitamin B12 deficiency manifesting as reversible leukoencephalopathy and ataxia. Neurology Asia, 26(1), 183-186.

5. NHS. Vitamin B12 or folate deficiency anaemia - Complications.