Antivirals are medications that keep viruses from reproducing inside the body. To date, relatively few effective antiviral drugs have been developed. It is difficult to create effective antiviral medications because viruses are tiny (smaller than bacteria and fungi) and use a cell’s metabolic pathways to replicate. This makes it more difficult to develop medications that attack the virus without also damaging the host cell.

However, some antiviral drugs are known to be effective. There are several types of antiviral drugs used to prevent or treat the various forms of influenza virus. Four antiviral prescription medications are commercially available to fight influenza viruses in the United States. These drugs are prescribed by a physician and work only against the flu and not other viruses (such as the viruses that cause common colds). The four types of flu antivirals are:

• amantadine (Symmetrel)
• rimantadine (Flumadine)
• oseltamivir (Tamiflu)
• zanamivir (Relenza)

Amantadine and rimantadine are medications called adamantane derivatives and are available in pill form. These drugs work against some, but not all, of the most severe forms of influenza viruses. They are thought to inhibit the activity of an influenza viral protein, which prevents the virus from replicating after it enters a cell.

Zanamivir (an inhaled powder) and oseltamivir (an oral drug) are medications known as neuraminidase inhibitors. These are a newer type of medication that works by blocking the active site of the influenza viral enzyme neuraminidase, which is found in the most severe influenza viruses. This enzyme normally breaks the bonds that hold new virus particles to the outside of an infected cell. This frees new viruses to infect other cells and spread infection. Neuraminidase inhibitors' ability to block this enzyme's activity prevents new virus particles from being released. This limits the spread of infection.

All four medications are known to be effective in modestly reducing the duration of flu symptoms (by about one day) when taken within two days after a person has been infected. They may also make infected people less contagious. Early indications are that oseltamivir may be effective in treating bird flu, but more study is needed before this can be declared to be the case.

These antivirals also can be taken as preventative measures to lessen the risk of flu infection. Patients generally take these medications for about 10 days when using them to prevent flu. According to the U.S. Centers for Disease Control and Prevention (CDC), viral resistance to amantadine and rimantadine has increased in the past several years, reducing the ability of these mediations to prevent influenza infections. The CDC recommends the use of oseltamivit and zanamivir, instread.

Experts generally advise against using antiviral medications in people who contract the flu, but who are otherwise healthy and at low risk of developing flu-related complications. In such cases, flu typically lasts for a few days before passing relatively quickly.

Antiviral medications typically are prescribed to control outbreaks of influenza in institutions with large numbers of people at high risk for complications associated with the flu. Examples include nursing homes or hospital wards. Typically, patients receive these drugs as stand-alone preventative medications or as protection against the flu until the flu vaccine begins to take effect (typically about two weeks after administration).

Other people who are at increased risk for flu-related complications and who may benefit from taking antiviral medications when they are infected with the flu include:

• Children 12 months to 23 months of age
  
  
• People 65 years of age and older
  
  
• People with chronic medical conditions (e.g., heart or lung disease, diabetes)
  
  
• Pregnant women

Antiviral drugs also may be prescribed in the event of a flu pandemic. A pandemic is an outbreak of an infectious disease that spreads throughout a country, over a continent or across the world and affects many people. The World Health Organization (WHO) has stockpiled antiviral drugs exclusively for this purpose. In the event of a pandemic, these drugs will be distributed in areas affected by the emerging pandemic virus with the goal of preventing or delaying the spread of the virus.

Although a course of antivirals can help prevent the flu, it is considered to be a far less effective prophylactic option than getting a dose of flu vaccine. In addition, physicians are reluctant to overprescribe antiviral drugs because this can lead to the appearance of drug-resistant flu viruses that are difficult to treat.

Flu vaccines are available each flu season from local health providers. Because influenza viruses mutate rapidly, a new form of the flu vaccine must be developed each year to protect people from the strains that experts believe will be most likely during that flu season.

While antiviral medications are most frequently used to treat influenza infections, they may also be used for other infections such as herpes simplex infection, respiratory syncytial virus (RSV) and hepatitis C.