What is this pill - with '' imprint?

I take a few meds daily but they are usually identified on both sides. I found a White, oval shaped pill with ONLY an "F" imprint on one side in my pocket. There is not imprint on the other side such as 54 commonly found on an anti anxiety med. I have no clue what it is and don't want to take it but if it is a current prescription that has changed its identifying mark and I need to take it I should. Can anyone assist?