cystitis- would erythromycin work?

[margonbread]

I have had cystitis since thursday evening.
I have had 6 satchets of stuff, drank loads of cranberry juice but it is still here.

I have a full course of erythromycin (250mg) that i was prescribed for something else but didn't take. They are in date ---would they do the trick or do i need to go to GP/walk-in clinic for a different anti-biotic?

thanks

surely you could have gotten the correct anti b via out of hours yesterday though?