Vitamin B12 shots are given to alleviate the symptoms of vitamin B12 deficiency, such as anemia, fatigue, and compromised immune system. Generally, they make people feel better, but they can also have some unpleasant side effects.

Most of the side effects are minor. By far the most common side effect is discomfort at the injection site. This can usually be relieved by applying ice packs or by taking an over-the-counter pain medication such as Tylenol.

Other common, but generally mild, symptoms include diarrhea and/or nausea, headaches, and joint pain. If these symptoms don’t fade with time, or if they become bothersome, tell your doctor. 

The very few people who have serious allergies to vitamin B12 injections experience much more pronounced side effects. While uncommon, if they occur, they require immediate medical attention.

Cardiac side effects can include the sensation that your heart is pounding or beating too fast, palpitations, chest pain, and tightness in the chest. Some people also experience respiratory symptoms such as wheezing, uncontrollable coughing or trouble breathing.

Other symptoms that are cause for immediate concern include muscle pain and cramping, especially in the legs; swelling of the hands and feet; confusion or dizziness; and hives, itching, rashes, or red patches on the face or body.

Again, these serious side effects are very rare. Most people who receive vitamin B12 injections benefit from them and experience few or no side effects. Nevertheless, just in case you happen to be one of those unfortunate individuals with an allergy, it’s a good idea to know what warning signs to watch for.