If you are currently being treated with any of the following medications, you should not use vitamin B12 supplements without first talking to your healthcare provider.

Antibiotics, Tetracycline
Vitamin B12 should not be taken at the same time as the antibiotic tetracycline because it interferes with the absorption and effectiveness of this medication. Vitamin B12 either alone or in combination with other B vitamins should be taken at different times of the day from tetracycline. (All vitamin B complex supplements act in this way and should therefore be taken at different times from tetracycline.)

In addition, long-term use of antibiotics can deplete vitamin B levels in the body, particularly B2, B9, B12, and vitamin H (biotin), which is considered part of the B complex.

Anti-ulcer Medications
The body's ability to absorb vitamin B12 is decreased when taking stomach acid-reducing medications such as omeprazole, lansoprazole, ranitidine, cimetidine, or antacids that are often used to treat gastroesophageal reflux, ulcers or related symptoms. This interference is most likely to occur as a result of prolonged use (more than one year) of these medications.

Chemotherapy Medications
Blood levels of vitamin B12 may be reduced when taking chemotherapy medications (particularly methotrexate) for cancer.

Metformin for diabetes
Blood levels of vitamin B12 may also be reduced when taking metformin for diabetes.

Phenobarbital and Phenytoin
Long-term treatment with either phenobarbital and phenytoin for seizure disorders may interfere with the body's ability to use vitamin B12.