Vitamin B12 is considered to be a safe supplement. At the recommended dosages, toxicity is highly unlikely. Since it is a water soluble vitamin, the body will excrete any excess through the urine. As such, no upper limit of intake has been established yet.[1]
It seems that a high vitamin B12 level could increase the risk of certain cancers and decrease the risk of others. One study[2] found an association between elevated B12 levels and prostate cancer; however, it did not establish a causal relationship. A cohort study in 2017 found an increased lung cancer risk for men who smoked and took relatively high doses of vitamin B12. While this association doesn’t necessarily mean B12 caused the increased rate of lung cancer, it is certainly worth considering before supplementing in this population group.[3]
Conversely, another study found that a higher vitamin B12 intake was associated with a lower risk of colorectal cancer.[4]
In cardiovascular disease, the data also remains uncertain. A 2024 meta-analysis found that increased B12 levels were associated with an increase in cardiovascular disease mortality. However, this study looked at endogenous vitamin B12, not vitamin B12 intake, and this might indicate that B12 levels rise in the presence of cardiovascular disease.[5] Another study found that vitamin B12 had little impact on cardiovascular disease outcomes.[6] More studies will need to be done to assess supplemental vitamin B12 as a risk factor for cardiovascular disease.