The dihydrate form of azithromycin, an orally bioavailable azalide derived from erythromycin, and a member of a subclass of macrolide antibiotics, with anti-bacterial activity. Upon oral administration, azithromycin reversibly binds to the 23S rRNA of the 50S ribosomal subunit of the bacterial ribosome of susceptible microorganisms, thereby inhibiting the translocation step of protein synthesis by preventing the assembly of the 50S ribosomal subunit. This inhibits bacterial protein synthesis, inhibits cell growth and causes cell death.