Derek Kaiser said: "It is necessary to take the full dosage schedule the effectiveness may be decreased and the bacteria may become resistant to antibiotics. There is a possibility of rare but very serious reactions to Zithromax, and serious skin diseases. Azithromycin is effective against susceptible bacteria causing infections of the upper or lower airways, skin or soft tissue, or ears. Azithromycin works by stopping the growth of bacteria. Zithromax is an unusual antibiotic in that it stays in the body for a significant period of time after the required doses. "