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I did a search on the archives for Zithromax and found lots of mixed
messages, so I called Pfizer (1-800-438-1985) myself.  I was assured by the
medical specialist that the product (I asked about the 250mg tablets) are
gluten-free.  They are not lactose free.

She also explained that GI symptoms (such as diarhhea and flatulence) are
reported with this anti-biotic, but affirmed there is no gluten in the
medication.