Wiktionary

1. Z-pak(Noun)

   A package of six pills, each one containing 250 mg of the antibacterial azithromycin.

   The pills in a Z-pak are to be taken as follows: ingest two pills on the first day, at the same time, then ingest one pill a day for the next four days.

Origin: Initial of Zithromax (Pfizer's proprietary name for azithromycin) and pack.