B12 Stopped Working? Why It Happens and What to Do Next

Almost every week, a patient tells me their B12 worked for a few weeks and then stopped. By then, they've usually tried everything to get it working again. But very few people know why it stopped in the first place.

Many people describe the same pattern. A few weeks of feeling like themselves again, then that familiar sense of B12 working slowly fades. Yet most keep taking it, just in case they'd feel worse without it.

Meanwhile, the fatigue and brain fog gradually return. Some symptoms even keep getting worse. So they try higher doses, different forms and more frequent shots. Or start hunting for a missing cofactor.

Usually, none of it brings that first effect back. But it's not because the body "got used to it." It comes down to what B12 actually does, and where its role ends.

B12 helps your body make healthy red blood cells to carry more oxygen. Also, it helps your cells break down certain fats and proteins for energy. 

Therefore, if your levels are low, topping them up can make a difference to how you feel. But once your levels are normal, adding more doesn't give you extra energy. 

That's why B12 only helps if you're actually deficient.

Every cell in your body needs NAD⁺ (nicotinamide adenine dinucleotide) to turn the food you eat and the oxygen you breathe into energy. No NAD⁺, no energy, no matter how much food or oxygen you take in.

Besides helping produce energy, NAD⁺ is also used by your cells to repair DNA damage. With age, that damage builds up, so your body burns more NAD⁺ just to keep up with repairs. At the same time, it gets worse at recycling NAD⁺ back into a usable form. So you're losing it faster and replacing it slower.

The result: By 40, roughly half your NAD⁺ is gone. And it happens whether you eat well, sleep well, or exercise. ³

NMN (nicotinamide mononucleotide) is a molecule your body uses as the raw material to build NAD⁺. ⁴

It naturally occurs in foods like broccoli, cabbage, edamame, and avocado. But the amounts are tiny, far too little to restore the NAD⁺ you've lost with age.

The effect of NMN was proven in several studies on humans.

In one study from 2022, healthy adults took 250 mg of NMN every day for 12 weeks. Within just 4 weeks, their NAD⁺ levels rose significantly and stayed elevated for the rest of the study. Without any side effects. The researchers concluded that taking NMN is a safe and simple way to boost your NAD⁺. ⁴

B12 has its place. But what matters more is whether your cells still have enough NAD⁺ to produce more energy. This gets harder to keep up as you age. That's why more and more of the people I speak with are now turning to NMN.

When they ask me where to start, I always tell them to pay attention to quality and dosage. NMN isn't regulated like a medicine, so what's in the capsule can vary a lot between brands. That's why I only recommend pure, third-party tested options with a clinically studied dose. The one I've tried that meets these standards, is PureLane NMN.

Your energy didn't just disappear, and it won't come back overnight either. But now you understand what's really happening inside your cells. So you can stop guessing and start supporting your body the right way.