Side effects of vitamin B-12 include stomach pain, nosebleeds, chest pain, dizziness and irregular heartbeat, according to Drugs.com. Additional side effects include ringing in the ears, chest tightness, difficulty breathing, weakness and weight gain.
Vitamin B-12 treats physical conditions such as low levels of B-12 in the blood and pernicious anemia, notes WebMD. It is present in a wide variety of foods, including meat, fish and dairy products, and the body requires it for the brain, nerves, blood cells and other parts to function correctly. Although it is generally safe for most people who take the recommended daily amount, women who are pregnant or breastfeeding should not take amounts that exceed 2.6 and 2.8 micrograms per day respectively because studies have not tested the safety of larger doses.