There are no known drug and food interactions with B12. However, this information is always changing and you should always tell your doctor about any medications you are taking, both prescription and over-the-counter. You should also tell your doctor if you are already taking over-the-counter vitamin B supplements. Your doctor may adjust your dose of B12 or may tell you to stop taking the OTC supplement. B12 may upset your stomach, in which case you should take it with a meal, unless your doctor directs you to do otherwise.