What are the side effects of vitamin b12 injections?

Physicians administer B12 injections to patients with B12 deficiencies and the symptoms associated with the deficiency, such as anaemia and fatigue. Injections of B12 are often given to patients who tend to get sick frequently and remain sick from common illnesses for extended periods of time.

B12 injections are often painful, as they are injected deep under the skin into a muscle.

Common Side Effects

There are many common side effects associated with B12 injections that need to be reported to a health care provider only if they persist for an extended amount of time. Common side effects include headaches, upset stomach, pain or other abnormal sensation at the injection site, swelling at the injection site, nausea, diarrhoea and joint pain.

Uncommon Side Effects

Uncommon side effects are much more serious than the common side effects of B12 injections, and such side effects should be reported to a physician or nurse to be evaluated for seriousness. Uncommon and dangerous side effects include rapid heartbeat, chest pain, flushed face, muscle cramps and weakness, difficulty breathing and swallowing, dizziness, confusion, rapid weight gain, tight feelings in the chest, hives, skin rashes, shortness of breath when there is no physical exertion and unusual wheezing and coughing.


Medical conditions and other medications, both prescription and non-prescription, can result in side effects when the condition or medication interacts with the B12 injection. If a patient is pregnant or breast feeding, has kidney or liver disease, an iron or folic acid deficiency, food or drug allergies or other medical conditions, he or she should report these conditions to the health care provider prescribing the B12 injections. Medications that should be avoided while taking B12 injections, in order to reduce side effects, are alcohol, antibiotics, potassium supplements and various vitamins.


If other unusual experiences follow a B12 injection, they should be reported to a doctor or nurse, who will determine if the side effect is common or more serious. Patients should advise health care providers of their allergies and current medications in order to minimise the side effects of B12 injections.

Expert Insight

Doctors and health care providers should review all of the patient's medical history, allergies and current medications before beginning the administration of B12 injections. Many health care providers ask that a patient remain at the facility for an hour following the injection to ensure that no immediate, serious side effects occur outside of immediate care.


The above is not a full list of side effects or interactions; a health care provider should always be consulted for any side effects that are not explained to a patient before a B12 injection is administered. Only a nurse or health care provider can safely and legally administer B12 injections, and a need for the injection must be reported in the patient's medical charts as well as a background check for safety of the treatment before injections can be administered.