Vitamin B12 injections are generally safe, but some people may experience side effects, such as pain, redness, or swelling at the injection site, mild headaches, dizziness, nausea, or itching
"From a consumer-buyer-beware standpoint, anytime you consume or inject something where you don't know what you're taking, there's the possibility of adverse health effects, and that's concerning," said Matt Fedoruk, USADA's chief science officer
Concordant clinical presentation, suggestive neuroimaging findings, and/or biochemical evidence raise suspicion for the correct diagnosis, but acute neurologic conditions consistent with vitamin B12 deficiency should be treated without waiting for definitive biochemical confirmation (69)
B12 is often found in animal-based foods, so vegetarians and vegans may also be at risk