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Why Don’t Antibiotics Kill Viruses?

Medically reviewed by L. Anderson, PharmD. Last updated on June 21, 2019.

Antibiotic Resistance Overview

Overuse and inappropriate prescribing of antibiotics worldwide is leading to the global healthcare issue of antibiotic resistance. However, the issue of antibiotic resistance can be confusing for many patients. You may be told you cannot use an antibiotic for a viral infection because they are ineffective and may lead to “antibiotic resistance”.

Why don’t antibiotics kill viral infections, and how can overuse of an antibiotic lead to “antibiotic resistance”?

  • Antibiotics cannot kill viruses because viruses have different structures and replicate in a different way than bacteria.
  • Antibiotics work by targeting the growth machinery in bacteria (not viruses) to kill or inhibit those particular bacteria.
  • When you think about it structurally, it makes sense that an antibiotic could not work to kill a virus with a completely different set of replicating “machinery”.

Illnesses caused by viruses

Most viral illnesses do not need special medication and are “self-limiting”, meaning your own immune system will kick in and fight off the illness. However, this can take time; a cough and cold can last from 7 to 10 days and the flu might keep you down for 2 weeks or more.

If you come down with a viral illness, you should rest, drink plenty of fluids and treat symptoms - like fever or aches and pains - with proper doses of pain and fever relievers, like over-the-counter (OTC) acetaminophen or ibuprofen, or as directed by your doctor. If you are diagnosed with a viral illness such as a cough, cold or sore throat, and your symptoms worsen or do not clear up within 10 days, be sure to contact your doctor.

  • In some viral infections, such as the flu, shingles (herpes zoster), or chicken pox (varicella) your doctor may decide to prescribe an antiviral drug to shorten your infection and to help prevent complications. Antivirals need to be taken early in the infection - usually in the first 24 to 48 hours - to be most effective.
  • In complicated or prolonged viral infections, bacteria may invade as well, and cause what is known as a “secondary bacterial infection”. In these cases, your doctor may prescribe an antibiotic, if one is needed, to kill the specific invading bacteria. The antibiotic is not being prescribed to treat the virus.

Viruses are structurally different from bacteria. Viruses live and replicate inside of a human cell, they cannot live outside of this environment. Viruses insert their genetic material into a human cell’s DNA in order to reproduce.

Antibiotics cannot kill viruses because bacteria and viruses have different mechanisms and machinery to survive and replicate. The antibiotic has no “target” to attack in a virus. However, antiviral medications and vaccines are specific for viruses. Vaccines stimulate your own immune system to produce antibodies, which then can “recognize” the virus to inactivate it before it can cause disease. The best way to help prevent the flu, shingles and chickenpox is with a vaccine.

Can I treat a cold with an antibiotic?

Using an antibiotic for a virus, like a cold:

  • will not cure the virus
  • won’t help you feel better
  • will not prevent others from catching your virus
  • will be a waste of your money.

Many bacterial infections do require an antibiotic; however, the type of antibiotic will vary based on the type of infection. An antibiotic either prevents bacterial growth (bacteriostatic) or kills bacteria outright (bactericidal).

It is very important not to share your antibiotics with someone else. For example, amoxicillin (a penicillin-type drug) can be used to treat a bacterial strep throat but will not work for some common pneumonias or bladder infections. While you may mean well, the bacteria causing their infection may not be susceptible to your prescribed antibiotic. In turn, those bacteria may not die and their infection can worsen. Plus, the person you share your antibiotic with may experience side effects or serious allergic reactions from your drug.

Common illnesses caused by bacteria

How does antibiotic resistance occur?

In general terms, antibiotic resistance can occur when bacteria learn to “fight off” the antibiotic.

  • Antibiotics work by interfering with the bacterial cell wall and prevent bacteria from making copies of themselves. However, many of these drugs have been widely used for a long period of time, overused, or used inappropriately.
  • Antibiotics are designed to kill specific bacteria but, over time, bacteria learn to adapt to the medicine, making the drugs less effective.
  • Bacteria fights back against a drug in many ways:
    • by strengthening their own cell walls
    • by producing enzymes that can inactivate the antibiotic
    • by helping out their fellow bacteria who are less able to "fight off" the antibiotic.

Why do I need to finish my antibiotic?

It's important to finish all of your prescribed antibiotic, even if you feel 100% better. Antibiotic resistance can occur if you do not finish all of your medication. Resistant bacteria are stronger and harder to kill, and need more potent medications.

In the worse-case scenario of antibiotic resistance, there may be no antibiotics that are effective for your serious antibiotic-resistant infection, hospitalization may be needed, and the infection can be life-threatening. According to the CDC, each year, at least 2 million people in the U.S. become infected with bacteria that are resistant to antibiotics and at least 23,000 people die as a direct result of these infections.

How can vaccines help?

Many infections can be prevented by following the recommended vaccine schedule as proposed by the CDC, so be sure to keep up-to-date with your vaccines and those of your children. Your doctor and pharmacist can provide more information about important vaccines for you and your family.

See Also


  1. US Centers for Disease Control and Prevention (CDC). Antibiotic Prescribing and Use in Doctor’s Offices. May 16, 2019. Accessed June 21, 2019 at
  2. Up to Date. Patient education: What you should know about antibiotics (The Basics). Accessed June 29, 2015 at
  3. Microbiology 101: Why Antibiotics Don’t Kill Viruses. Dr. Barry Dworkin. January 28, 2003. Accessed June 21, 2019 at
  4. NPS MedicineWise. Antibiotics Don’t Kill Viruses. Accessed June 21, 2019 at

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.