Why Don’t Antibiotics Kill Viruses?
Medically reviewed by L. Anderson, PharmD. Last updated on Mar 8, 2017.
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”. But why don’t antibiotics kill viral infections, and how can overuse of an antibiotic lead to “antibiotic resistance”?
Simply put, 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”.
Examples of illnesses that are caused by viruses:
- Most sore throats
- Most coughs, colds and runny noses
- Acute sinusitis
- Acute bronchitis
- Some eye or ear infections
- Respiratory syncytial virus (RSV)
- Flu (influenza)
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. 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 acetaminophen or ibuprofen, or as directed by your doctor. If you are diagnosed with a viral illness like a cough, cold or sore throat, and your symptoms 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. Sometimes, 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, but the antibiotic is not being prescribed for your virus.
Viruses are structurally different from bacteria. Viruses live and replicate inside of a human cell, they cannot live outside of a human cell. 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 go out and “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.
Using antibiotics for a virus:
- Will not cure the infection
- Won’t help you feel better
- Will not prevent others from catching your virus
Many bacterial infections will 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).
For example, amoxicillin (a penicillin-type drug) can be used to treat a strep throat but will not work for some common pneumonias or bladder infections. This is one reason why it is very important not to share your antibiotics with someone else. 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 the infection can worsen. Plus, the person you share your antibiotic with may unnecessarily experience side effects from your drug.
Examples of illnesses caused by bacteria:
- Strep throat
- Bacterial pneumonia
- Whooping cough (pertussis)
- Many skin infections
- Some ear or eye infections
- Some sinus infections, but usually these are viral
- Bacterial meningitis
- Urinary tract infections
Antibiotics work by interfering with the bacterial cell wall and preventing bacterial replication. However, these drugs have been widely used for a long period of time, and the bacteria the antibiotics are designed to kill have adapted to them, making the drugs less effective. Bacteria can fight back in many ways - by strengthening their own cell walls, by producing enzymes that can inactivate the antibiotic, and even by helping out their fellow bacteria who are less able to be resistant to the antibiotic. They learn to “fight off” the antibiotic, so to speak.
That’s one reason why 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. 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 each year as a direct result of these infections.
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 your and your family.
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- CDC: Centers for Disease Control and Prevention. Brochure for Parents: Snort. Sniffle. Sneeze. No Antibiotics Please. April 17, 2015. Accessed June 29, 2015 at https://www.cdc.gov/getsmart/community/materials-references/print-materials/parents-young-children/snort-sniffle-sneeze-b.html
- Up to Date. Drug Resistance and Choice of Therapy. June 17, 2015. Accessed June 29, 2015 at https://www.uptodate.com/contents/antibiotic-studies-for-the-treatment-of-community-acquired-pneumonia-in-adults
- Microbiology 101: Why Antibiotics Don’t Kill Viruses. Dr. Barry Wodkin. January 28, 2003. Accessed June 29, 2015 at http://www.drbarrydworkin.com/articles/medicine/infectious-disease-articles/microbiology-101-why-antibiotics-dont-kill-viruses/
- NPS MedicineWise. Antibiotics Don’t Kill Viruses.
Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.