Side effects are possible for any food, supplement, or medication, and everyone reacts differently to different products. Vitamin C, for example, may promote loosening of the bowels if taken in excess, and too much niacin (B3) may cause a disagreeable flushing. Experiencing unpleasant side effects of a supplement is by no means enjoyable, and it's best for consumers to do their research before diving into any supplement approach. Generally, the worst side effects for vitamins–including B vitamins–are diarrhea, nausea, rashes, and vomiting. 
Does vitamin B12 have any side effects? Here's what you need to know.
Let's quickly give a brief overview of how vitamin B12 is absorbed. High doses of vitamin B12 have a low absorption rate when taken orally. Excess vitamins are what you want to watch out for. To survive the digestive process, B12 must connect with intrinsic factor before it can enter the system. The amount of B12 that does get absorbed is directly related to the amount of intrinsic factor available.  For this reason, oral ingestion of large amounts of B12 is highly unlikely to reach the small intestine for absorption. Even sublingual absorption (absorption through tissue of the mouth) doesn't reach levels which would be of cause for concern.
Symptoms of Deficiency
Deficiency can result for a couple of reasons. Inadequate dietary consumption of foods containing B12 such as red meat, fish, or dairy products–an issue for many vegetarians and vegans–can lead to deficiency. An inability to digest B12 as a result of low levels of intrinsic factor–for either genetic reasons or a result of physical damage to the stomach or small intestine–can also cause deficiency.
Common symptoms include numbness in the hands and feet, moodiness, memory problems, dementia, poor red blood cell formation, impaired DNA function, and fatigue.
B12 Side Effects
B12 occurs naturally in three forms: methylcobalamin, adenosylcobalamin, and hydroxocobalamin. Cyanocobalamin was created years ago in a lab using cyanide as a means for stabilization, and many low-quality supplements include this form. Injections are commonly used for individuals suffering from an inability to digest and absorb B12. Many individuals who have chosen a vegetarian or vegan lifestyle become B12 deficient as a result of eliminating natural dietary sources; still, many meat eaters today are also surprisingly deficient.
Some side effects have been reported for patients receiving cyanocobalamin injection. These include itching, acne, nausea, diarrhea, vascular thrombosis, headache, nervousness, and dizziness. Based on studies involving individuals, it appears 1 mg of cyanocobalamin via injection does not create any notable side effects. 
No Upper Limit Safety Recommendations Exist
No adverse or toxic reactions for vitamin B12 have occurred in individuals through dietary consumption or supplement use. The US Food and Nutrition Board have not established any upper safety limits for B12 in healthy people. Although you should always check with your doctor before starting a new supplement routine, supplementation with B12 is considered safe.
-Dr. Edward F. Group III, DC, ND, DACBN, …read more