Most people take methylated B12 without any problems. But like any supplement, it can cause side effects in some people. The good news? Most side effects are mild and go away on their own.
This guide will walk you through everything you need to know about methylated B12 side effects. You’ll learn what’s normal, what’s not, and when to call your doctor.
What Is Methylated B12?
Methylated B12 is the active form of vitamin B12. Your body uses it right away without having to change it first.
Unlike cyanocobalamin, which is a man-made form of B12, methylcobalamin occurs naturally in foods like fish, meat, eggs, and milk. This makes it easier for your body to absorb and use.
Many people choose methylated B12 because:
- It works faster in your body
- It’s better for people with gene mutations like MTHFR
- It helps with nerve health and brain function
Want to learn more basics? Check out our guide on what is methylated B12.
Common Side Effects That Are Usually Mild
Most side effects from methylated B12 are nothing to worry about. Common side effects include headache, upset stomach or throwing up, loose stools, and belly pain.
These usually happen when you first start taking it. Your body just needs time to get used to the supplement.
Digestive Problems
Some people with vitamin B12 deficiency who take methylcobalamin experience nausea, vomiting, diarrhea, loss of appetite, and headaches.
These tummy troubles are temporary. They often clear up within a few days as your body adjusts.
If your stomach still hurts after a week, try:
- Taking it with food
- Lowering your dose
- Switching to a different time of day
Headaches and Dizziness
Injectable vitamin B12 can cause common side effects like mild diarrhea, itching, nausea and vomiting, dizziness, headaches, and hot flashes.
Headaches are more common with injections than pills. They usually go away after the first few doses.
Drink plenty of water. This can help reduce headaches when you start taking B12.
Skin Reactions
Some people notice mild skin changes like:
- Itching or tingling
- Small red bumps
- Mild redness at the injection site (if using shots)
Common side effects when taking methylcobalamin include headache, itching, swelling, anxiety, restlessness, and involuntary or uncontrolled movements.
These skin reactions are usually harmless. But if they get worse or spread, call your doctor.
Serious Side Effects That Need Attention
While rare, some side effects need immediate medical help. Signs of a serious allergic reaction include rash, hives, itching, red or swollen or blistered or peeling skin with or without fever, wheezing, tightness in the chest or throat, trouble breathing or swallowing or talking, unusual hoarseness, or swelling of the mouth, face, lips, tongue, or throat.
Allergic Reactions
You should not use methylcobalamin if you are allergic to vitamin B12 or cobalt.
Cobalt allergy is estimated to affect 1 to 3 percent of the general population. Since B12 contains cobalt, people with cobalt allergies can have serious reactions.
Signs of a severe allergic reaction:
- Hives or rash that spreads quickly
- Swelling of your face, lips, tongue, or throat
- Trouble breathing or swallowing
- Feeling dizzy or like you might pass out
Call 911 right away if you have these symptoms. Anaphylaxis is a severe reaction to a trigger such as an allergy and is potentially life threatening.
Heart and Breathing Problems
Serious side effects include shortness of breath and very bad dizziness or passing out.
Some people also report:
- Fast or irregular heartbeat
- Chest tightness
- Sudden weight gain
- Swelling in your legs or feet
These could be signs of a bad reaction. Stop taking B12 and see a doctor right away.
Very High Doses and Cancer Risk
Taking too much B12 for a long time might cause problems. Extremely high vitamin B12 levels have been linked to the development of lung and prostate cancer.
This doesn’t mean B12 causes cancer. But very high levels in your blood might be a warning sign to check with your doctor.
Most people don’t need mega-doses of B12. Stick to the recommended amount unless your doctor says otherwise.
Overmethylation: When You Get Too Much
Some people feel worse when they take methylated B12. This might be a sign of overmethylation.
Overmethylation is thought to occur when the body has more methyl groups than it needs or can use, which may lead to symptoms like anxiety, fatigue, poor concentration, panic attacks, sleep disorders, and sensitivities to environmental toxins.
What Overmethylation Feels Like
Common signs include:
- Feeling anxious or jittery
- Racing thoughts
- Trouble sleeping
- Feeling “wired but tired”
- Heart racing or pounding
- Irritability
Individuals who experience over methylation often struggle with a cluster of symptoms such as high anxiety, difficulty sleeping, and a sense of overstimulation.
Who Gets Overmethylation?
Those who are overmethylating have copious amounts of SAMe and easily assimilate activated forms of folate and vitamins B6 and B12.
You might be more likely to have this problem if you:
- Have certain gene variations (like slow COMT)
- Take high doses of methylated vitamins
- Feel worse on methylfolate or methyl B12
- Have low histamine levels
If this sounds like you, talk to your doctor about trying a different form of B12. Individuals with slow COMT may benefit from non-methylated forms of B12 such as hydroxocobalamin, which is less likely to overstimulate methylation pathways.
Learn more about different B12 types in our article on methylcobalamin vs cyanocobalamin.
Drug Interactions You Should Know About
Methylated B12 can interact with some medicines. Always tell your doctor what you’re taking.
Diabetes Medications
There is clear evidence that proton-pump inhibitors, H2-receptor antagonists, and metformin can reduce serum vitamin B12 concentrations by inhibiting the absorption of the vitamin.
If you take metformin for diabetes, you need more B12. Evidence suggests that long-term and high-dose metformin therapy impairs vitamin B12 status.
Your doctor might want to:
- Check your B12 levels every year
- Give you B12 supplements
- Increase your B12 dose
Stomach Acid Reducers
Do not take methylcobalamin without medical advice if you are using medicines that reduce stomach acid such as cimetidine, omeprazole, lansoprazole, Nexium, Prevacid, Prilosec, or Zantac.
These drugs make it harder for your body to absorb B12 from food. But methylated B12 supplements can still work because they don’t need stomach acid to be absorbed.
Antibiotics and Other Drugs
Some medicines can affect how B12 works:
- Chloramphenicol (an antibiotic)
- Colchicine (for gout)
- Potassium supplements
Drug interactions can change how your medication works or increase your risk for serious side effects. Keep a list of all your medicines and show it to your doctor.
Who Should Not Take Methylated B12?
Certain individuals should avoid using methylcobalamin, including those with a known allergy to methylcobalamin or any of its components, individuals with Leber’s disease, a hereditary optic neuropathy, as methylcobalamin may worsen the condition.
People with Leber’s Disease
This is a rare eye disease that runs in families. Cyanocobalamin should be used cautiously in patients with Leber optic nerve atrophy because it can increase disease severity.
If you have Leber’s disease, talk to your eye doctor before taking any B12 supplement.
Pregnant or Nursing Women
B12 is important during pregnancy. But always check with your doctor first.
Tell your doctor if you are pregnant or breastfeeding. Your doctor can help you pick the right dose and form.
People with Low Potassium
Hypokalemia resulting in death has occurred during vitamin B12 therapy as a result of increased red blood cell requirements during hematopoiesis.
If you have low potassium in your blood, your doctor needs to check and fix this before you start B12. This is rare but serious.
How to Take Methylated B12 Safely
Following these simple tips can help you avoid side effects:
Start with a Lower Dose
High-dose supplements aren’t always a good idea and can cause major side effects if not treated with respect.
Begin with a smaller amount. You can always increase it later if needed. Most people do well with 500 to 1,000 mcg per day.
Take It at the Right Time
Water-soluble vitamins are better absorbed on an empty stomach, so the usual time of taking methylcobalamin will be in the morning as the first dose, half an hour before lunch, and 2 hours post a meal.
But if it upsets your stomach, take it with food instead. It’s better to take it with food than to skip it because of tummy troubles.
Avoid Alcohol
Avoid drinking large amounts of alcohol as heavy drinking can make it harder for your body to absorb methylcobalamin.
A drink now and then is fine. But too much alcohol can stop B12 from working right.
Store It Properly
Store methylcobalamin in a dry and cool place, preferably at room temperature between 20°C and 25°C, keep it away from direct contact with light, heat, and air.
Keep it in a cool, dry spot away from sunlight. Don’t store it in the bathroom where it’s humid.
When to Call Your Doctor
Most side effects are mild. But you should call your doctor if:
- Side effects last more than a week
- Symptoms get worse instead of better
- You have signs of an allergic reaction
- You feel chest pain or can’t breathe well
- Your heart races or beats irregularly
- You have severe dizziness or pass out
If you notice any adverse effects after you start taking vitamin B12 supplements, you should discontinue use immediately and seek medical attention if your symptoms get worse or are severe.
Don’t be afraid to speak up. Your doctor can adjust your dose or switch you to a different form of B12.
Can Methylated B12 Cause Weight Gain?
Methylcobalamin is not known to cause weight gain as it is a water-soluble vitamin and any excess amounts are typically excreted in the urine.
Despite many processes and trials involving vitamin B12, there is little evidence to suggest that it has any effect leading to weight gain.
Some people worry about this, but B12 doesn’t make you gain weight. In fact, fixing a B12 deficiency might give you more energy to be active.
The Bottom Line on Methylated B12 Side Effects
Methylated B12 is safe for most people. Common side effects like upset stomach, headaches, and mild itching usually go away quickly.
Serious side effects are rare. But if you have trouble breathing, severe swelling, or chest pain, get help right away.
The key to avoiding problems:
- Start with a lower dose
- Tell your doctor about all your medicines
- Watch for signs of overmethylation
- Don’t take it if you’re allergic to cobalt
- Call your doctor if side effects don’t go away
Want to learn how B12 can help you? Read about the benefits of methylated B12 and see if it’s right for you.
Final Thoughts
Methylated B12 helps millions of people feel better every day. It boosts energy, supports nerve health, and keeps your brain sharp.
Yes, side effects can happen. But for most people, the benefits far outweigh the risks. If you’re thinking about taking methylated B12, talk to your doctor first. They can help you find the right dose and form for your needs.
Remember: your body is unique. What works for someone else might not work for you. Pay attention to how you feel. And don’t be afraid to ask questions or make changes if something doesn’t feel right.
With the right approach, methylated B12 can be a safe and helpful part of your health routine. Ready to try it? Check out our shop for high-quality methylated B12 supplements.