[allykatt2005]

Can taking Zithromax (Azithromycin) while on the birth control pill Seasonale cause an unexpected period in women?

[pharmd06]

unlikely.

but remotely possible. pretty much all antibiotics carry a warning of decreasing the effectiveness of birth control pills. (effectiveness meaning you have inc risk of getting pregnant)
this is bc the antibiotic jump starts the livers metabolism of the birth control and it is also theorized that the gut bacteria, that help reabsorb excreted estrogens, are killed. so you essentially have 2 factors decreasing hormone levels.

this can also cause menstration because the levels of hormones are in a delicate balance and if the levels decrease enough or suddenly change menstration may result. consult your prescribing doctor if you do menstrate for instructions on whether you should continue the current pack.