Common antibiotics and what they are used for (research guide)
MedlinePlus, the NIH's drug information service, says common antibiotics such as amoxicillin, doxycycline and ciprofloxacin each treat certain infections caused by bacteria, and each one has its own list of people who should not take it. The CDC says antibiotics do not work on viruses such as colds and flu, and should be taken exactly as prescribed, never shared and never saved for later.
The CDC says more than 9.3 million drug-resistant infections happen in the US each year, and more than 39,000 people die as a result.
- For research and education only
- 12 common antibiotics at a glance
- Antibiotics do not work on viruses
- Why the wrong antibiotic, old pills or a partial supply are dangerous
- Allergy danger and signs of anaphylaxis
- How to get an emergency supply legally
- Fish and pet antibiotics
- Signs an infection is an emergency
- Gear
For research and education only
For research and education only. Not medical advice. This page does not give doses and does not tell you which antibiotic to take for any illness. Only a doctor, nurse practitioner or pharmacist who has examined you can do that. Every fact below links to the source that states it.
12 common antibiotics at a glance
Each drug name links to its MedlinePlus page from the National Institutes of Health. The "used for" column gives only what MedlinePlus lists. Many of these drugs are prescribed for other infections too.
| Drug | Drug class | What MedlinePlus says it is used for | Who should not take it, and allergy notes | Major warnings |
|---|---|---|---|---|
| Amoxicillin | Penicillin-like antibiotic | Certain bacterial infections such as pneumonia, skin and urinary tract infections. Also used with other medicines to get rid of H. pylori, a germ that causes ulcers. | Is a penicillin drug (ACAAI). Tell your doctor about any drug allergy, kidney or liver disease, or mononucleosis. | Hives, wheezing, or swelling of the face, throat or tongue. Blistering or peeling skin. Severe watery or bloody diarrhea, which can start 2 months or more after treatment. |
| Amoxicillin and clavulanic acid | Penicillin-like antibiotic plus a beta-lactamase inhibitor | Certain bacterial infections, including infections of the ears, lungs, sinuses, skin and urinary tract. | Contains amoxicillin, a penicillin drug (ACAAI). Your doctor may tell you not to take it if it caused liver problems before. | Liver warning signs: yellow skin or eyes, pain in the upper right stomach. Allergy signs. Watery or bloody stools. |
| Penicillin V potassium | Penicillin | Certain bacterial infections such as pneumonia, scarlet fever, and ear, skin, gum, mouth and throat infections. Also used to stop rheumatic fever coming back. | Penicillin allergy. Tell your doctor about kidney disease, asthma or other allergies. | Rash, hives, wheezing or throat swelling. Severe diarrhea up to 2 months or more after treatment. Joint pain. Fever or sore throat that comes back. |
| Cephalexin | Cephalosporin antibiotic | Certain infections caused by bacteria. MedlinePlus says it will not work for colds, flu or other viral infections. | Allergy to cephalexin. Tell your doctor about all drug allergies. | Watery or bloody diarrhea during treatment or 2 or more months after. Allergic reaction. Fever, sore throat or chills that come back. Hallucinations. |
| Doxycycline | Tetracycline antibiotic | Bacterial infections, and certain infections spread by ticks, lice, mites and infected animals. Also used to prevent malaria, and for acne and rosacea. | Should not be used in children under 8 except in certain cases. Can harm an unborn baby. Can permanently stain teeth if used in pregnancy or under age 8. Does not contain penicillin (ACAAI). | Makes skin sensitive to sunlight. Call a doctor right away for trouble breathing or swallowing, unusual bleeding or bruising, or joint pain. |
| Azithromycin | Macrolide antibiotic | Bacterial infections. Also used to treat and prevent disseminated Mycobacterium avium complex (MAC), a lung infection in people with weak immune systems. | Your doctor will probably tell you not to take it if it caused jaundice or liver problems before. Tell your doctor about any history of a prolonged QT interval. Does not contain penicillin (ACAAI). | Prolonged QT is a rare heart problem that may cause irregular heartbeat, fainting or sudden death. Get help for a fast, pounding or irregular heartbeat. |
| Ciprofloxacin | Fluoroquinolone | Treats or prevents certain bacterial infections. Also used for inhalation anthrax and plague. | Myasthenia gravis: your doctor may tell you not to take it. Tell your doctor about seizures, kidney disease and all drug allergies. | MedlinePlus important warning: tendinitis and tendon rupture during treatment or for months after (highest risk over age 60), nerve damage that may not go away, worse muscle weakness in myasthenia gravis, and serious mental effects including thoughts of self-harm. |
| Levofloxacin | Fluoroquinolone | Certain bacterial infections. Also used to prevent anthrax after breathing in the germs, and to treat and prevent plague. | Myasthenia gravis: your doctor may tell you not to take it. Tell your doctor about all drug allergies. | MedlinePlus important warning: tendinitis and tendon rupture during treatment or for months after, nerve damage that may last, and severe breathing trouble or death in people with myasthenia gravis. |
| Co-trimoxazole (sulfamethoxazole and trimethoprim) | Sulfonamide | Certain bacterial infections, and travelers' diarrhea. | Can harm an unborn baby. Tell your doctor about any allergy to it and about low folic acid. Your doctor may tell you not to take it if you have certain blood conditions. | Rash, peeling or blistering skin. Unusual bruising or bleeding. Yellow skin or eyes. Shortness of breath. Makes skin sensitive to sunlight. Severe diarrhea up to 2 months or more after. |
| Nitrofurantoin | Antibiotic | Urinary tract infections. | Should not be taken in the last month of pregnancy. Your doctor will probably tell you not to take it if you have had kidney disease, or jaundice or liver problems from it. | Lung problems (cough that will not stop, trouble breathing). Liver signs (yellow skin or eyes, dark urine). Numbness or tingling in fingers and toes. |
| Metronidazole | Nitroimidazole antimicrobial | Certain bacterial infections, including some sexually transmitted diseases and bacterial vaginosis. | Do not drink alcohol, or use products with alcohol or propylene glycol, during treatment and for at least 3 days after. Tell your doctor if you take disulfiram or stopped it in the past 2 weeks. | MedlinePlus important warning: has caused cancer in lab animals. Seizures, confusion, poor coordination. Numbness, burning or tingling in hands or feet. |
| Clindamycin | Lincomycin antibiotic | Certain types of bacterial infections. Will not work for colds, flu or other viral infections. | Allergy to clindamycin. Does not contain penicillin (ACAAI). | MedlinePlus important warning: can cause overgrowth of dangerous bacteria in the large intestine and a life-threatening colitis. Peeling or blistering skin. Yellow skin or eyes. |
MedlinePlus says to tell your doctor or pharmacist about every allergy and every other medicine you take before starting any of these. The allergy notes on penicillin come from the American College of Allergy, Asthma and Immunology (ACAAI).
Antibiotics do not work on viruses
The CDC says antibiotics only treat certain infections caused by bacteria, such as strep throat, whooping cough and urinary tract infections.
- The CDC says antibiotics do not work on viruses, such as colds, flu and bronchitis.
- Most sore throats (except strep throat) and most chest colds are caused by viruses, according to the CDC.
- The CDC says antibiotics will not make you feel better if you have a virus.
The side effects listed in the next section can still happen when the drug does nothing for a virus.
Why the wrong antibiotic, old pills or a partial supply are dangerous
- Delay. The CDC says taking the wrong medicine for a future illness may delay correct treatment and can cause severe side effects. That is why it says not to save antibiotics for later or share them.
- Side effects. The CDC lists rash, dizziness, nausea, diarrhea and yeast infections, plus C. diff infection, which causes diarrhea that can lead to severe colon damage and death, and severe, life-threatening allergic reactions.
- Expired pills. The FDA says expired medicines can be less effective or risky, and that weakened antibiotics can fail to treat infections, leading to more serious illness and antibiotic resistance.
- Resistance. The CDC says resistance happens when germs develop the ability to defeat the drugs designed to kill them, and it speeds up when antibiotics pressure bacteria to adapt.
- Take it exactly as prescribed. That is the CDC rule, start to finish.
Allergy danger and signs of anaphylaxis
Anaphylaxis is a severe allergic reaction. MedlinePlus says symptoms often develop within seconds or minutes. The ACAAI says penicillin allergy symptoms can start within an hour but can also come days later.
Signs to know (MedlinePlus):
- Trouble breathing or swallowing, wheezing, chest tightness
- Swelling of the face, eyes or tongue
- Hives and itching
- Dizziness or lightheadedness, a racing heart, slurred speech
- Stomach pain, nausea, vomiting or diarrhea
- Passing out
What to do: MedlinePlus says call 911 right away, use an epinephrine injector or nasal epinephrine immediately if one is available, and give nothing by mouth if the person is having trouble breathing.
Do you really have a penicillin allergy? The CDC says about 10% of US patients report one, but when clinicians checked, fewer than 1% of them were truly allergic. Ask your doctor about allergy testing now, while help is easy to reach, so your records are right before an emergency.
How to get an emergency supply legally
- Ask your own doctor. The CDC says to talk to your doctor or pharmacist about building an emergency supply of your medicines. That conversation is the legal route for any prescription drug, antibiotics included.
- Travel is the clearest example. The CDC's travel packing list includes a travelers' diarrhea antibiotic and copies of all prescriptions. The CDC Yellow Book, written for clinicians, discusses prescribing a standby antibiotic for self-treatment, and says the harms (drug-resistant germs, C. difficile and allergic reactions) must be weighed against the benefit.
- Keep the paperwork. The CDC says to keep a written list of your medicines, doses, medical supplies and allergies.
- Uninsured in a disaster? See the federal Emergency Prescription Assistance Program on our medicines in a disaster page.
Fish and pet antibiotics
The FDA says drugs like ciprofloxacin and amoxicillin are sold to treat bacterial diseases in fish. Here is what else it says:
- These products have not been approved, conditionally approved or indexed by the FDA, and have not been evaluated for safety and effectiveness.
- They may not meet the FDA's standards for purity and potency, and may not be properly labeled.
- The FDA says it does not know how they have been handled and stored.
- The FDA says animal drugs should not be used to treat people, and that you should never take animal drugs.
Signs an infection is an emergency
The CDC says sepsis is the body's extreme response to an infection, and a life-threatening medical emergency. A person with sepsis might have one or more of these signs:
- Clammy or sweaty skin
- Confusion or disorientation
- Extreme pain or discomfort
- Fever, shivering or feeling very cold
- High heart rate or weak pulse
- Shortness of breath
Where There Is No Doctor, the Hesperian guide for places far from a doctor, says a wound infection is spreading if it causes fever, a red line runs up from the wound, or the lymph nodes become swollen and tender. It says brown or gray liquid, black skin, or air bubbles around a wound may mean gangrene, and to get medical help fast.
The CDC says about 1.7 million US adults develop sepsis each year, and at least 350,000 adults who develop it die in the hospital or after going to hospice. Treat these signs as an emergency and get the person to medical care by any safe means.
Common mistakes
- Taking an antibiotic for a cold or flu. The CDC says antibiotics do not work on viruses.
- Saving leftover antibiotics for later or sharing them. The CDC says not to, because the wrong medicine can delay correct treatment.
- Using expired antibiotics. The FDA says weakened antibiotics can fail to treat infections and lead to resistance.
- Choosing an antibiotic yourself from a chart like this one. The table is for research. Only a clinician can match a drug to an infection and to you.
- Buying fish or pet antibiotics. The FDA says they are not evaluated for safety or effectiveness and should never be taken by people.
- Ignoring hives, swelling or wheezing after a new medicine. MedlinePlus says anaphylaxis can develop within seconds or minutes. Call 911.
- Waiting out confusion, a racing heart or shortness of breath with an infection. The CDC says these can be signs of sepsis, a medical emergency.
Gear for this
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Medication list holder or wallet card
The CDC says to keep a written list of your medicines, doses and allergies. Keep it with your emergency supply.
Medical alert ID bracelet
Listed on the CDC travel packing list. Tells responders about a drug allergy if you cannot.
Digital thermometer
On the CDC packing list. Fever is one of the CDC sepsis signs and a sign a wound infection is spreading.
Weekly pill organizer
Helps you take prescribed medicine exactly as directed, which the CDC asks of anyone on antibiotics.
Questions people ask
Do antibiotics work on colds or the flu?
No. The CDC says antibiotics do not work on viruses, such as colds, flu and bronchitis, and will not make you feel better if you have a virus.
Are fish antibiotics the same as human antibiotics?
Some contain the same drugs, such as ciprofloxacin and amoxicillin, but the FDA says they are not approved, have not been evaluated for safety or effectiveness, and may not meet standards for purity and potency. The FDA says you should never take animal drugs.
Can I keep leftover antibiotics for an emergency?
The CDC says do not save antibiotics for later or share them, because the wrong medicine can delay correct treatment and cause severe side effects. Ask your doctor about a proper emergency supply instead.
Sources
- MedlinePlus: Amoxicillin
- MedlinePlus: Amoxicillin and Clavulanic Acid
- MedlinePlus: Penicillin V Potassium
- MedlinePlus: Cephalexin
- MedlinePlus: Doxycycline
- MedlinePlus: Azithromycin
- MedlinePlus: Ciprofloxacin
- MedlinePlus: Levofloxacin
- MedlinePlus: Co-trimoxazole
- MedlinePlus: Nitrofurantoin
- MedlinePlus: Metronidazole
- MedlinePlus: Clindamycin
- MedlinePlus: Anaphylaxis
- CDC: Antibiotic Do's and Don'ts
- CDC: About Antimicrobial Resistance
- CDC: Clinical Features of Penicillin Allergy
- CDC: About Sepsis
- CDC: Prescriptions, Prepare Your Health
- CDC: Pack Smart
- CDC Yellow Book 2026: Travelers' Diarrhea
- FDA: Ornamental Fish Drugs and You
- FDA: Don't Be Tempted to Use Expired Medicines
- ACAAI: Penicillin Allergy
- Hesperian: Where There Is No Doctor, Chapter 10, First Aid
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- C-07Pandemic and disease outbreak: what to have at homeA 2-week supply, bought slowly. The checklist health departments recommend.
General safety information only, not medical or professional advice. Rules can change, so check the linked sources and follow your local officials. As an Amazon Associate I earn from qualifying purchases.