The medications you described: Zithromax (Z-pac) and Cipro are not interchangable. The are from entirely different classes of antibiotics.
HOWEVER, Both Zithromax and Cipro (but not Cipro XR the sustained formulation of Cipro) are FDA approved for use in "lower respiratory infections"
Since you have never had a problem with Cipro, and you already have the medication, and since you have had a problem with Z-pac - it is probably reasonable to use the Cipro instead.
If you are concerned - you could always call your physician - they should have someone all call. I suspect that your doctor and/or the on-call doctor would probably tell you the same thing.
Suggestion for the future - the next time you see your doctor - remind him that you had a problem with Z-pac, so that he can make a note in your chart.
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