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. There are three major types of influenza:
- Types A and B. The most severe forms of the virus, they constantly change and different strains circulate around the world every year.
- Type C. Causes either a very mild illness or has no symptoms at all.
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 influenza type A viruses, but not influenza type B viruses. They are thought to inhibit the activity of an influenza A viral protein, which prevents the virus from replicating after it enters a cell.
Zanamivir (an inhaled powder) and oseltamivir (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 both influenza A and influenza B 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. Amantadine and rimantadine are between 70 percent and 90 percent effective in preventing symptoms of influenza A illness, according to the U.S. Centers for Disease Control and Prevention (CDC).
The individual antivirals may not be effective against all types of influenza strains. For instance, in 2006 a high proportion of circulating flu viruses was found to be resistant to amantadine and rimantadine. For this reason, the CDC recommended against using these antivirals as preventative or treatment medications during the 2006 flu season.
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 type A influenza mutates 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.
Types and differences of antiviral drugs
In addition to treating influenza, antivirals also may be used to treat other medical conditions. For example, interferons are synthetic versions of a substance naturally produced by cells in the body that help fight infections and tumors. Interferons are often used to treat hepatitis B and C and certain types of cancer.
Other antivirals include:
- Acyclovir. Drug used to heal sores or blisters caused by varicella (chickenpox), herpes zoster (shingles) and genital herpes. Acyclovir also can prevent outbreaks of genital herpes in people who are infected with the virus. In some cases, regular treatment with acyclovir has kept people symptom-free for as long as a decade, and with few reported side effects.
- Ganciclovir. Drug used to treat or prevent cytomegalovirus (CMV), a virus that infects cells and causes them to become enlarged.
- Ribavirin. Drug used with interferon to treat hepatitis C. It helps stop the virus that causes hepatitis C from spreading inside the body. Ribavirin is also sometimes used to treat viral hemorrhagic fevers and severe acute respiratory syndrome (SARS).
- Vidarabine. Drug used to treat virus infections of the eye caused by herpes infection. It sometimes causes increased sensitivity to light that can be reduced by wearing sunglasses and avoiding too much exposure to bright light.
Certain antiviral drugs also are used specifically to treat patients with HIV and AIDS. These drugs are known as antiretroviral medications because they attack the retrovirus (virus that contains RNA and reverse transcriptase) that causes AIDS. They include protease inhibitors, nucleoside-analog reverse transcriptase inhibitors and non-nucleoside reverse transcriptase inhibitors. Currently, there are more than two dozen retroviral drugs used to treat HIV/AIDS.
Other types of antivirals
There are several different kinds of antiviral agents that are prescribed to fight viral skin infections. These medications can be divided into systemic antivirals and topical antivirals.
Systemic antivirals are usually given orally in pill form. However, they sometimes may be administered intravenously (into a vein). This is most likely in situations where the patient is suffering from a suppressed immune system (due to HIV infection, organ transplantation or another condition), is unable to swallow oral medication, has impaired memory or mental capacity or does not have quick access to a hospital should complications arise during drug therapy.
Systemic Antivirals Generic Name Brand Name(s)
Topical antivirals are usually available in various forms, including creams, ointments, gels, patches and sprays. In addition, most topical antivirals can be further subdivided into one of three major classes:
- Cytodestructive drugs (destroys infected tissue)
- Immunoenhancing drugs (enhances immune system)
- Viricidal drugs (destroys viruses)
Topical Antivirals Generic Name Brand Name(s)
Conditions treated with antiviral medications
Most viruses that cause skin infections can be divided into several major groups. These include:
- Human herpesvirus (HHV). This family of viruses includes strands of the herpes simplex virus that cause cold sores (herpes simplex virus type 1) and genital lesions (herpes simplex virus type 2), and the herpes zoster virus that causes chickenpox and shingles. Human herpesviruses 6 and 7 cause a childhood illness called roseola infantum that can cause high fever and rash. Systemic antiviral agents are typically used to treat HHV.
- Human papillomavirus (HPV). A group of viruses that cause various types of warts (papillomas). These include common warts, foot warts, flat warts, genital warts and filiform warts. Some types of warts require no treatment, whereas others may need treatment with topical or systemic antiviral agents.
- Poxvirus. A group of viruses that include cowpox, monkeypox, and smallpox. One of the most destructive viral infections, smallpox has been eradicated throughout the world. Since 1978, only isolated, laboratory-acquired forms of the disease have been reported. It is usually treated with a vaccine that is effective up to four days after exposure, although the drug cidofovir has shown promise in the laboratory as a potential treatment.
- Human immunodeficiency virus (HIV). HIV is the virus that causes acquired immune deficiency syndrome (AIDS). Skin lesions that appear as a result of this infection are often the first sign that the patient has HIV. These lesions often result from a separate infection with HHV, papillomavirus or other infectious agents. Systemic antiviral agents are typically used to treat HIV. Numerous other medications are available that specifically treat HIV.
Conditions of concern with antivirals
Experts generally advise against using the antiviral zanamivir in people with underlying respiratory disease, including asthma or chronic obstructive pulmonary disease (COPD). Some people with such conditions who have taken zanamivir have experienced bronchospasm.
Patients with kidney disease may need to have their dosage of oseltamivir adjusted. In addition, people who are allergic to any of these antiviral drugs or their ingredients should not take them.
In laboratory and limited clinical studies, there have been no reported interactions of the antiviral medication neuraminidase inhibitors with other drugs.
Patients should closely consult with their physician about whether certain conditions preclude taking specific antiviral agents.
- Side effects
Potential side effects
In general, the newer antiviral neuraminidase inhibitors (e.g., oseltamivir) are less likely to cause side effects than the older flu drugs. Side effects associated with antiviral medications include:
- Loss of appetite
In addition, adverse central nervous system (CNS) effects have been noted with use of some antivirals. Such side effects include lightheadedness, difficulty concentrating, nervousness, insomnia and seizures in patients with pre-existing seizure disorders.
There have been reports of serious respiratory problems in some people who use zanamivir.
In some cases, taking antiviral drugs can lead to the appearance of drug-resistant flu viruses. For example, when either amantadine or rimantadine is used for therapy, drug-resistant flu viruses may appear in about one-third of patients, according to the National Institutes of Health (NIH).
Laboratory studies indicate that influenza A and B viruses can develop resistance to zanamivir and oseltamivir. Studies are ongoing to see if this effect is duplicated in patients who take neuraminidase inhibitor medications.
Side effects associated with antivirals other than those used to treat the flu include:
- Changes in vision
- Dry, irritated or itchy skin
- Hair loss
- Memory loss
- Pain, particularly in the joints, bones or muscles
- Painful or irregular menstruation (period)
Patients are urged to closely follow their physician’s instruction for taking antiviral medications. This will help both lower the risk of side effects for the patient and help prevent the development of drug-resistant flu viruses in the public at large.
Drug or other interactions
Patients should consult their physician before taking any additional prescriptions, over-the-counter medications, nutritional supplements or herbal medications. Of particular concern to individuals using antiviral agents are:
- Cimetidine (used to treat gastrointestinal disorders)
- Probenecid (used to treat chronic gout or gouty arthritis)
- Theophylline (used to treat lung problems resulting from various disorders)
- How to use
Pregnancy use issues
The U.S. Food and Drug Administration (FDA) has classified most antiviral agents as belonging in either category B (which means that either animal studies have shown no risk to an animal fetus, or human studies have shown no risk to a human fetus) or category C (which means animal studies have shown a risk to animal fetuses, but risk has not been adequately studied in humans).
Patients should discuss the potential risks and benefits of using antiviral agents during pregnancy and breastfeeding with their physicians.
Child use issues
None of the antivirals used to treat influenza has been approved for use in children younger than 1 year of age. Children infected with the flu who are between 12 months and 23 months of age are most likely to benefit from antiviral medications.
Amantadine is approved for treating and preventing uncomplicated influenza A virus infection in children who are 1 year of age or older. Rimantadine is approved for preventing, but not treating, such infections in children. However these medications may not be effective every year. For example, in 2006 the U.S. Centers for Disease Control and Prevention (CDC) recommended against the use of these medications because the active strains of influenza were resistant to these drugs.
Zanamivir is approved for preventing influenza A and B virus infections in people 5 years of age and older and for treating uncomplicated influenza virus infection in people 7 years of age and older who have not had symptoms for more than two days.
Oseltamivir is approved for treating uncomplicated influenza virus infection in people 1 year of age or older who have not had symptoms for more than two days. A pediatric liquid formulation of this medication is available. Oseltamivir also is approved for preventing influenza A and B in people 1 year of age and older.
Questions for your doctor regarding antiviralsPatients may wish to ask their doctor the following questions related to antivirals:
- Should I take an antiviral drug if there is a flu outbreak in my community?
- Do I have a condition that prohibits me from taking an antiviral?
- Why do you advise me to take an antiviral?
- Which antiviral do you suggest? Why?
- What potential risks are associated with my antiviral?
- What options do I have aside from taking an antiviral?
- Should other members of my family take an antiviral drug?
- If I have had the flu vaccine, will I need an antiviral as well?
- What are the potential risks to my child after taking an antiviral?
- Do you think the benefits outweigh these risks?