Will warfarin cause low potassium levels? I take a 20 meq tablet of Potassium CL ER twice a day, 400 mg. magnesium oxide twice a day, and 5 mg. warfarin once a day. I felt weak like I was going to faint, went to ER and my potassium level was 2.5? Why? I would like to know. I eat a good diet--banana, fruits, vegetables every day.