Enter Your Email Address. CLICK HERE to learn more about from eMedTV.com. Relenza ® (zanamivir) is a prescription medication used to treat and prevent the flu (influenza). When taken at the first signs of the illness, the drug shortens the duration of flu symptoms. When taken before a person has the flu, such as during an outbreak, Relenza can help prevent the flu. Unlike other flu medicines, this one is inhaled.