Why Don’t Antibiotics Kill Viruses?

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on June 2, 2025.

Key Points

Overuse and inappropriate prescribing of antibiotics worldwide is leading to the global healthcare issue of antibiotic resistance. 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”? Some reasons are that:

• 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”.

List of common illnesses 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)
• COVID

Most viral illnesses do not need special medication and are “self-limiting”. This means that usually your own immune system will kick in and fight off the illness, but this can take time. A cough and cold can last from 7 to 10 days and the flu, RSV or COVID might keep you down for 1 to 2 weeks or more.

If you have tested positive for COVID-19, be sure to contact your doctor for further advice. Antiviral medications are now available for some high-risk groups with COVID-19, and vaccinations or boosters may be recommended.

If you come down with a viral illness, you should rest, drink plenty of fluids and treat symptoms such as fever or aches and pains. Treatment options include 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), COVID or chicken pox (varicella) your doctor may decide to prescribe an antiviral drug to shorten your infection and to help prevent complications. Antivirals typically need to be taken early in the infection - usually in the first 24 to 72 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”, such as bacterial pneumonia. In these cases, your doctor may prescribe an antibiotic, if one is needed, to kill the secondary invading bacteria. The antibiotic is not being prescribed to treat the virus.

How are viruses different from bacteria?

Viruses are structurally different from bacteria. Viruses live and replicate inside of a human cell and 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.

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, COVID, shingles and chickenpox is with a vaccine.

Can I treat a cold with an antibiotic?

Using an antibiotic for a virus, like a cold or the flu:

• will not cure the virus
• won’t help you feel better
• will not prevent others from catching your virus.

Many bacterial infections do require an antibiotic; however, the type of antibiotic will vary based on the type of infection and bacteria. 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 if you share your medicine with other people, the bacteria causing someone else's infection may not be susceptible to your prescribed antibiotic. In turn, those bacteria may not die and that person's infection can worsen. They may still be contagious to those around them, as well.

Plus, the person you share your antibiotic with may experience serious side effects or allergic reactions from your drug. Common antibiotic side effects include nausea, vomiting, diarrhea, vaginal yeast infections, and more. Overall, sharing any medicine with someone else is risky.

List of common illnesses caused by bacteria

• Strep throat
• Bacterial pneumonia
• Whooping cough (pertussis)
• Skin infections
• Ear or eye infections
• Sinusitis (usually viral but can become bacterial)
• Bacterial meningitis
• Tuberculosis
• Urinary tract infections

Learn more: What are the strongest antibiotics available?

How does antibiotic resistance occur?

According to the CDC's latest statistics, at least 2.8 million people in the U.S. each year become infected with bacteria that are resistant to antibiotics and at least 35,000 people die as a direct result of these infections.

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 drug less effective.
• Bacteria fight 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.
• In the worse-case scenario of antibiotic resistance, there may be no antibiotics left that are effective for your serious antibiotic-resistant infection, hospitalization may be needed, and the infection can be life-threatening.

Do I need to finish my antibiotic?

The bottom line: take your antibiotic for as long as your doctor tells you to take it.

Historically you may have been told to always finish all of your prescribed antibiotic, even if you feel 100% better. The thinking was that antibiotic resistance could occur if you stop short of your full course of therapy.

But some experts have suggested that long courses are not always needed and could actually be fueling the antibiotic resistance trend. The American College of Physicians (ACP) published a guideline in 2021 entitled "ACP Best Practice Advice: Shorter course of antibiotics may be appropriate for some common infections".

• Healthcare providers and patients should be aware that using antibiotics for shorter periods may have the same or better results compared with longer periods and can also help lower antibiotic resistance.
• The treatment advice from ACP centers on uncomplicated and common infections like bronchitis / COPD, pneumonia, urinary tract infection (UTI), and skin infections (cellulitis) in otherwise healthy patients.
• Length of treatment often depends on the antibiotic being used or the bacteria being treated, too. For example, for uncomplicated UTI in women fosfomycin is a medicine that can be given in one single dose, but trimethoprim–sulfamethoxazole (Bactrim, Septra) is given as a 3-day course.

Because there is no overall guiding principle, take your antibiotic for as long as your doctor tells you. Sometimes treatment length may need to be extended if the infection does not clear, if you have a compromised immune system, you are pregnant, or you recently had treatment for a bacterial infection.

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 and timing for you and your family.

Related: What are vaccines, and how do they work?

See also

• Antibiotic Resistance: The Top 10 List
• Antibiotics - Common Side Effects, Allergies and Reactions
• Antibiotics and Birth Control Pill Interactions
• Antibiotics for UTI Treatment
• Can You Drink Alcohol with Antibiotics?
• Middle Ear Infection FAQs (Acute Otitis Media)

Further information

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

Medical Disclaimer

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