Vitamin B12 is considered safe and non-toxic vitamin. There are no perceivable side effects of this vitamin and for this reason, no Tolerable Upper Intake Level for Vitamin B12 has been prescribed by The Institute of Medicine of the National Academy of Sciences.
An intake any one of the B-complex vitamins by itself for a sustained period of time can result in an imbalance of other important B vitamins in the human body. Because of drug interactions with other medicines, the potential for side effects although seemingly nonexistent can not be ruled out.
Antibiotics taken over a long period of time have an adverse effect on body stores of vitamin B12. A simultaneous intake of vitamin B12 with antibiotic tetracycline interferees with the absorption and effectiveness of this medicine. Therefore, it is recommended that both should be taken at different times of the day.
Anti-ulcer medications like Proton Pump Inhibitors (PPIs) and H2 receptor antagonists that are used to treat gastroesophageal reflux disease (GERD) and peptic ulcer disease slow down the release of gastric acid into the stomach. There may be some interference of this with vitamin B12, since gastric acid is required to release vitamin B12 from food prior to absorption.
While taking in Metformin for diabetes, vitamin B12 levels may reduced. Medication for Chemotherapy also has the same effect. Phenobarbital and phenytoin seizure disorders may also interfere with the body's ability to use vitamin B12.
During such situations, the ideal remedy would be to consult a physician and registered dietitian to discuss the best way to maintain vitamin B12 status when taking these medications. Therefore, it is pertinent to say that although there are no declared side effects, it is necessary to maintain caution particularly vis-à-vis vitamin B12's with other medicines.