Vitamin B12 is water-soluble, meaning excess amounts are typically excreted in urine rather than stored in the body. This makes toxicity rare, but very high doses can still have potential effects, especially in certain individuals.

1. Potential Risks of High-Dose B12 Supplementation
A. Acne and Skin Reactions
🔹 High doses (above 1,000 mcg daily) have been linked to acne breakouts and rosacea flares, especially in people prone to skin issues.
🔹 Likely due to B12 altering the balance of skin bacteria, leading to inflammation.

B. Effects on the Kidneys
🔹 Extremely high doses (e.g., 5,000 mcg+ daily) may stress the kidneys, especially in those with kidney disease.
🔹 Some studies have linked high B12 levels with worsening kidney function in people with diabetic nephropathy.

C. Increased Cancer Risk?
🔹 Some studies suggest high B12 blood levels (not necessarily from supplements) correlate with higher lung cancer risk, particularly in smokers.
🔹 Possible reason: Excess B12 might fuel rapid cell proliferation, which could be problematic if there are pre-cancerous cells.

D. Impact on Blood Pressure & Blood Vessels
🔹 High-dose B12 injections have been linked to rare cases of increased blood pressure and blood vessel dysfunction.
🔹 Mechanism not fully understood but might involve changes in nitric oxide levels.

E. Interaction with Other Nutrients
🔹 High-dose B12 can lower folate levels in some cases, affecting methylation balance.
🔹 Excess B12 with low folate may increase homocysteine, a marker linked to cardiovascular risk.

2. Who Should Be Cautious with High-Dose B12?

🚨 People with Kidney Disease → Reduced clearance can lead to excessively high B12 levels.
🚨 Smokers → Potential association with increased lung cancer risk.
🚨 Those with Acne or Rosacea → High doses may trigger breakouts.
🚨 People with Cancer or a History of Cancer → The role of B12 in cell growth makes high doses a potential concern.

3. How Much B12 Is Safe?
Daily Recommended Intake: ~2.4 mcg/day for adults.
Typical Supplement Doses: 500–1,000 mcg/day for deficiency treatment.
High-Dose Cases: 5,000 mcg/day or more (used in some clinical treatments but not for general supplementation).

💡 For most people, a daily dose of 500–1,000 mcg is safe if needed, but megadoses (>5,000 mcg) should be used with caution and under medical supervision.