Sometimes it is not safe to use certain medications at the same time. Some drugs can affect your blood levels of other drugs you take, which may increase side effects or make the medications less effective.
Tell your doctor about all your current medicines. Many drugs can affect Flagyl, especially:
an antidepressant;
asthma medication;
cancer medicine;
heart or blood pressure medication;
lithium or other antipsychotic medicine;
medicine to treat malaria, HIV, or other infection; or
a blood thinner - warfarin, Coumadin, Jantoven.
This list is not complete and many other drugs may interact with metronidazole. This includes prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible drug interactions are listed here.
 Flagyl drug interactions (more detail)
