What is the drug flecainide given for; what purpose? I thought it was a blood thinner but now told it isn t. I was diagnosed with atrial fib in August of this year and put on Coumadin and Flecainide. The Flecainide caused several side effects including but not limited to dizziness, fainting, chest pressure, weight gain, claminess, etc. After two weeks of relating this to my cardiologist was finally taken off of it this past Friday. How long before it is out of my system?