There are a couple other antibiotics that can be prescribed if NGU is persistent, Erythromycine or
Moxifloxacin. However, I would only use
Moxifloxacin as a last resort, as it can have some very harsh and lasting side effects. In addition, if it's fungal related (Trich) they can also treat you with an alternative regimen of metronizadole.
Return to your doctor and discuss persistent NGU with him and CDC guidelines for testing and treating:
http://www.cdc.