Injecting too much B12 can have adverse effects on the body. B12 is a water-soluble vitamin, which means that excess amounts are typically excreted through urine. However, in cases of excessive B12 injection, the body may struggle to eliminate the excess vitamin, leading to potential complications. Some common side effects of over-injecting B12 include diarrhea, nausea, vomiting, and stomach cramps. In rare cases, individuals may experience more severe symptoms such as chest pain, difficulty breathing, and allergic reactions. It is important to consult with a healthcare professional before starting any B12 injections to ensure proper dosage and minimize the risk of adverse effects.