Why Won't Antibiotics Help a Cold or the Flu?

Do antibiotics work for colds and the flu? No: both are caused by viruses, and antibiotics act on bacteria. Taking one for a viral illness does not shorten it, and it can cause side effects and add to antibiotic resistance. This article explains how viruses and bacteria differ, why a cold or flu can seem to call for an antibiotic, which warning signs suggest a bacterial complication, and what can help you feel better.

Do antibiotics work for colds and the flu?

When you feel awful, it is natural to want a prescription. But the common cold and influenza are viral infections, and antibiotics are designed to kill or slow bacteria. A medicine built for one kind of germ has no target in the other.

MedlinePlus, a service of the National Library of Medicine at NIH, notes on its Antibiotics page that antibiotics treat bacterial infections and do not work on viruses. Its Common Cold page describes colds as viral illnesses that usually improve on their own, with treatment aimed at easing symptoms.

Viruses and bacteria: what antibiotics actually target

Bacteria are living single-celled organisms. Some cause illnesses like strep throat, urinary tract infections and some pneumonias. Antibiotics work by interfering with structures or processes that bacteria need to grow or survive.

Viruses are much smaller and depend on your own cells to reproduce. They lack the structures antibiotics attack. MedlinePlus explains this difference on its Viral Infections page. Several different viruses can cause a cold, and influenza viruses cause the flu.

What a published review found about antibiotics for colds

Researchers have looked at this question directly. A Cochrane systematic review, Antibiotics for the common cold and acute purulent rhinitis, examined trials of antibiotics in people with colds. Its authors found no clear benefit of antibiotics for the common cold and more adverse effects than with placebo.

The flu works the same way. Antibiotics do not act on influenza viruses, so they are not a treatment for the flu itself. A clinician may discuss a prescription antiviral medicine, particularly for people at higher risk of complications, such as pregnant people, adults 65 and older, young children and people with chronic heart, lung, kidney or immune conditions. The CDC's Influenza Antiviral Medications: Summary for Clinicians advises starting treatment as soon as possible for people at higher risk, with the clearest benefit when it is started within 48 hours of symptom onset. If you are in a higher-risk group, contact a clinician early. Whether an antiviral is appropriate depends on your health and symptoms.

Does green mucus mean you need an antibiotic?

Many people believe that thick yellow or green mucus signals a bacterial infection. Mucus color can change during an ordinary cold because immune cells and other substances in the mucus change as your body fights the virus. Color alone is not a reliable way to tell viral from bacterial illness, so clinicians combine your history, an examination and sometimes testing.

A lingering cough causes similar confusion. Acute bronchitis, an inflammation of the airways, is often caused by viruses, and MedlinePlus notes the cough can last a few weeks. A cough that lasts longer, or comes with fever or shortness of breath, should be checked by a clinician.

The real costs of an antibiotic you don't need

Like all medicines, antibiotics carry risks. Taking one when it cannot help exposes you to those risks for no gain.

• Common side effects. Nausea, stomach upset, rash and diarrhea can occur. Side effects vary by drug and person. A mild new rash is worth a call to your clinician or pharmacist. A rash with blisters, peeling skin, sores in the mouth or eyes, fever or facial swelling can be a severe skin reaction and needs emergency care right away.
• Allergic reactions. Some people react to antibiotics with hives, swelling or trouble breathing. A serious allergic reaction is an emergency.
• Disruption of normal gut bacteria. Antibiotics affect helpful bacteria along with harmful ones. In some people this can allow an overgrowth of a germ such as Clostridioides difficile (C. diff), which can cause bloody diarrhea, or watery diarrhea with fever or belly pain, during or after a course. See our article on why antibiotics cause diarrhea for details.
• Antibiotic resistance. Each use of an antibiotic gives bacteria a chance to adapt. Resistant bacteria can make later infections harder to treat, for you and for others.
• Drug interactions. Some antibiotics interact with other medicines, including some blood thinners and birth control methods. Your pharmacist can check your list.

Every antibiotic label also carries its own cautions. Some drug classes have boxed warnings or specific cautions for certain groups, such as pregnancy or older adults. For example, fluoroquinolone antibiotics can cause tendon, nerve and mental-health effects, so ask your prescriber or pharmacist about the cautions for the one you are given.

When a cold or flu may have a bacterial complication

Viral illnesses can sometimes open the door to a bacterial infection. Examples include bacterial sinus infection, ear infection and, after influenza, bacterial pneumonia. In these cases an antibiotic may be appropriate, and a clinician decides. Many mild ear and sinus infections improve without antibiotics, so a clinician may recommend watchful waiting, with a plan for follow-up if symptoms worsen or do not improve. How that is handled depends on age, how severe the symptoms are and your other health conditions.

Patterns that may raise the question include:

• Symptoms that improve and then get clearly worse again.
• A fever that returns after it had gone away, or a fever that is not improving after several days.
• Significant ear pain, or ear pain in a young child that is getting worse.
• Severe sore throat with a fever and without cold symptoms such as a runny nose or cough, which may call for a strep test.
• Facial pain or pressure with thick nasal discharge that has not improved after about 10 days, or that gets worse after initial improvement, a pattern that clinicians often evaluate for bacterial sinusitis.
• Shortness of breath, fast breathing or chest pain or tightness, which can point to the lungs or heart and need evaluation the same day at the latest. Breathing trouble at rest, trouble speaking in full sentences, or chest pain or pressure that is severe, spreads to the arm or jaw, or comes with sweating needs emergency care.

Our articles on a sore throat that will not go away and on a dry cough that lasts for weeks cover those specific problems in more depth.

Fever in a very young baby deserves special mention. Pediatric guidance advises prompt medical evaluation of a rectal temperature of 100.4°F (38°C) or higher in a young infant, and a baby younger than 3 months should be assessed by a clinician or an emergency department, because a serious bacterial infection can be hard to recognize at that age. An older infant who is feeding poorly, has noticeably fewer wet diapers, is unusually sleepy or looks ill should be seen urgently, and the pediatrician should be called about any fever in a baby.

Dehydration is another reason to be seen. If you or your child cannot keep fluids down, or you notice very little urine, a dry mouth or unusual drowsiness, get same-day care.

What can help while you recover

Treatment for a viral cold or flu is mostly about comfort and support while the immune system does the work.

• Rest and fluids. Sleep and staying hydrated support recovery.
• Humidified air and saline. A cool-mist humidifier or saline nasal spray or rinse may ease congestion.
• Acetaminophen (Tylenol). It may ease fever and aches. It can harm the liver in excess and is hidden in many multi-symptom cold and flu products, so do not exceed the labeled dose or combine products that share it. Ask a pharmacist first if you have liver disease or drink alcohol regularly. Poison Control (1-800-222-1222) can help if someone may have taken too much.
• Ibuprofen and other NSAIDs. They may also ease fever and aches, and their labels carry a boxed warning for cardiovascular and stomach bleeding risk, so follow the label. They are not suitable for everyone, including people with kidney disease, stomach ulcers or bleeding, those taking blood thinners, and pregnant people in later pregnancy, so ask a clinician or pharmacist first if any of these apply.
• Fever medicine for babies and children. Do not give any fever medicine to a baby under 3 months unless a clinician directs it, because a fever at that age needs medical evaluation. Ibuprofen labels do not recommend use in infants under 6 months. Amounts for children are based on weight and age, so ask a pediatrician or pharmacist rather than estimating a dose yourself.
• Avoid aspirin and salicylates in children and teens. Do not give aspirin or salicylate-containing products (such as bismuth subsalicylate) to children or teenagers with a cold, flu or flu-like illness unless a clinician directs it. They are linked to Reye syndrome, a rare but serious condition affecting the brain and liver. Read the active ingredients on every combination, stomach and multi-symptom product, since salicylates can appear under names such as aspirin, acetylsalicylic acid, salicylate or subsalicylate, and ask a pharmacist if unsure.
• Over-the-counter cold products. MedlinePlus explains on its Cold and Cough Medicines page that these products ease symptoms but do not cure the infection. Many contain several active ingredients, so check labels carefully for duplicated ingredients, including acetaminophen and salicylates. Combination products that contain a decongestant can raise blood pressure or heart rate, so if you have heart disease or high blood pressure, ask a pharmacist before using one.
• Prescription antivirals for flu. A clinician may consider one for certain patients with influenza. Ask your clinician whether it is appropriate for you and review the label cautions with your pharmacist.

For children, product choice and amounts depend on age, and some cold products are not appropriate for young children. Labels on many over-the-counter cough and cold medicines say not to use them in children under 4, so do not give them to young children unless a clinician advises it. New confusion, repeated vomiting or unusual sleepiness in a child after flu needs emergency care, and Reye syndrome is one possible cause. Follow the product label and check with your child's pediatrician or pharmacist. Our article on over-the-counter cough and cold medicine in young children covers this.

Talking with your clinician about antibiotics

If your clinician says you probably have a viral illness and an antibiotic is not likely to help, that is a considered decision, not a brush-off. You can ask what symptoms to watch for and when to come back.

A few habits reduce risk for you and others:

• Take an antibiotic only when it has been prescribed for your current illness.
• If you are prescribed one, ask your prescriber or pharmacist how to take it and what to do about a missed dose. Do not change the timing or amount on your own, and if side effects bother you, call your prescriber or pharmacist before stopping.
• Do not use leftover antibiotics or share them with others. They may be the wrong drug for the new illness.
• Tell your clinician about any prior allergic reaction to an antibiotic, and about any severe or bloody diarrhea during or after a course.
• Wash your hands, cover coughs, stay home when you are feverish, and talk with your clinician about vaccines that fit your age and health.

When to Seek Medical Care