hello, I have recently had a urine test and tested positive for ngu, been prescribed metronidazole, ciprofloxacin & prescribed myself with doxycycline tablets after research on the web. 1 day after i start medication I notice a rash in the head of my penis, it doesn't itch or irritate me, If I hadn't of seen it I wouldn't of knew it was there? can this be a side effect of by medication, or any other sti?