Can you mix nitrofurantoin and levofloxacin or are they antagonists?

You *may*, but ... The only time I would see using these at the same time is in the unfortunate circumstance that you were treating a urinary tract infection with the nitrofurantoin and a different infection with the levofloxacin - a highly unlikely circumstance. There's no reason to be using the two together to be treating the same infection. When it comes to antibacterials, never ever use more than you need.
Seebelow. These 2 drugs are not usually used together. Nitrofurantoin is a urinary antiseptic that does not achieve sufficient tissue levels to treat an invasive infection. It carries many potentially severe toxicities to the lung, liver, bone marrow and nerves so i seldom prescribe it. If you are taking Levofloxacin you don't need to take nitrofurantoin.