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Overview of Antibiotics


Brian J. Werth

, PharmD, University of Washington School of Pharmacy

Reviewed/Revised Jun 2022 | Modified Sep 2022
Topic Resources

Antibiotics are grouped into classes based on their chemical structure. However, antibiotics within each class often affect the body differently and may be effective against different bacteria.

Classes of antibiotics include the following:

Carbapenems, cephalosporins, monobactams, and penicillins are subclasses of beta-lactam antibiotics, a class of antibiotic characterized by a chemical structure called a beta-lactam ring.

Other antibiotics that do not fit into the classes listed above include chloramphenicol Chloramphenicol The antibiotic chloramphenicol is used mainly to treat serious infections due to the few bacteria that are resistant to other antibiotics but are still susceptible to chloramphenicol. Its use... read more , clindamycin Clindamycin Clindamycin belongs to a class of antibiotics called lincosamides. Clindamycin is used to treat serious bacterial infections, including some infections that are resistant to other antibiotics... read more , daptomycin Daptomycin The antibiotic daptomycin is used to treat many serious bacterial infections, such as those caused by gram-positive bacteria, including those that are resistant to many other antibiotics. Daptomycin... read more , fosfomycin Fosfomycin Fosfomycin is an antibiotic that has a unique chemical structure. It is used mainly to treat bladder infections caused by Escherichia coli (E. coli) or Enterococcus faecalis. It... read more , lefamulin Lefamulin Lefamulin is an antibiotic that works by interfering with the bacteria's production of the proteins it needs to grow and multiply. Lefamulin is used to treat community-acquired pneumonia that... read more , metronidazole Metronidazole and Tinidazole Metronidazole is an antibiotic used to treat pelvic, abdominal, soft-tissue, gum, and tooth infections and abscesses in the lungs or brain. It is also the preferred drug for certain protozoal... read more , mupirocin Mupirocin Mupirocin is an antibiotic that is used to treat impetigo and some other bacterial skin infections and to eliminate staphylococci from the nose. Mupirocin works by interfering with the bacteria's... read more , nitrofurantoin Nitrofurantoin Nitrofurantoin is an antibiotic that is used only to prevent or treat uncomplicated bladder infections. How nitrofurantoin works is not fully understood, but it disrupts several bacterial processes... read more , and tigecycline Tigecycline Tigecycline is the only antibiotic in an antibiotic class called glycylcyclines, which are related to tetracyclines. Tigecycline works by preventing bacteria from producing proteins they need... read more .

Selecting an Antibiotic

Each antibiotic is effective only against certain types of bacteria. In selecting an antibiotic to treat a person with an infection, doctors evaluate which bacteria are likely to be the cause. For example, some infections are caused only by certain types of bacteria. Sometimes one antibiotic is predictably effective against all of the bacteria that are most likely to be causing an infection and so further testing may not be needed.

If infections may be caused by many different types of bacteria or by bacteria that are not predictably susceptible to antibiotics, a laboratory is asked to identify the infecting bacteria Diagnosis of Infectious Disease Infectious diseases are caused by microorganisms, such as bacteria, viruses, fungi, and parasites. Doctors suspect an infection based on the person's symptoms, physical examination results,... read more from samples of blood, urine, or tissue taken from the person ( see Diagnosis of Infectious Disease Diagnosis of Infectious Disease Infectious diseases are caused by microorganisms, such as bacteria, viruses, fungi, and parasites. Doctors suspect an infection based on the person's symptoms, physical examination results,... read more ). The infecting bacteria are then tested for susceptibility to a variety of antibiotics. Results of these tests usually take a day or two and thus cannot guide the initial choice of antibiotic if the infection needs to be treated immediately. In such cases, doctors typically start treatment with an antibiotic that is effective against the bacteria most likely to be causing the infection. When test results are back, doctors change the antibiotic if needed.

Antibiotics that are effective in the laboratory do not necessarily work in an infected person. The effectiveness of the treatment depends on

These factors may vary from person to person, depending on other drugs being taken, other disorders present, and the person’s age.

In selecting an antibiotic, doctors also consider the following:

  • The nature and seriousness of the infection

  • The status of the person's immune system (how well it can help the drug fight the infection)

  • The drug’s possible side effects

  • The possibility of allergies or other serious reactions to the drug

  • The cost of the drug

Doctors also consider how hard it may be for people to take antibiotics for the entire time prescribed and complete the full course of treatment. People may find it more difficult to complete treatment if the drug must be taken very often or only at specific times (such as before meals, during meals, or after meals).

Combinations of antibiotics may be needed to treat the following:

  • Severe infections, particularly during the first days when the bacteria's susceptibility to antibiotics is not known

  • Certain infections caused by bacteria that rapidly develop resistance to a single antibiotic

  • Infections caused by more than one type of bacteria if each type is susceptible to a different antibiotic

Antibiotic Resistance

Bacteria, like all living organisms, change over time in response to environmental challenges. Because of the widespread use and misuse of antibiotics (when antibiotics are not taken as prescribed), bacteria are constantly exposed to these drugs. Although many bacteria die when exposed to antibiotics, if antibiotics are not taken appropriately, some bacteria survive and develop resistance Antibiotic resistance Bacteria are microscopic, single-celled organisms. They are among the earliest known life forms on earth. There are thousands of different kinds of bacteria, and they live in every conceivable... read more to the drugs’ effects. For example, 50 years ago, Staphylococcus aureus (a common cause of skin infections) was very sensitive to penicillin. But over time, strains of this bacteria developed an enzyme able to break down penicillin, making the drug ineffective. Researchers responded by developing a form of penicillin that the enzyme could not break down, but after a few years, the bacteria adapted and became resistant to this modified penicillin. Other bacteria have also developed resistance to antibiotics.

Medical research continues to develop drugs to combat bacteria. But people can help prevent the development of resistance in bacteria by

  • Understanding that antibiotics are used to treat bacteria, not viral infections (such as the common cold or the flu), and that doctors do not prescribe antibiotics for these viral infections

  • Taking antibiotics exactly as directed, including the correct dose, numbers of times per day, and number of days (it is important to take antibiotics for the full number of days prescribed, even if a person is feeling better)

Did You Know...

  • If a virus is causing the infection, taking antibiotics is useless and can contribute to the development of resistance in bacteria.

Taking Antibiotics

For less severe infections, antibiotics can often be taken by mouth from the start.

Spotlight on Aging: Antibiotics

When doctors prescribe antibiotics for older people, they may prescribe a lower dose than usual because the kidneys tend to function less well as people age. In such cases, the kidneys may not be able to eliminate antibiotics from the body as effectively, increasing the risk of side effects. (See also Aging and Drugs Aging and Medications Medications, the most common medical intervention, are an important part of medical care for older people. Without medications, many older people would function less well or die at an earlier... read more Aging and Medications .)

Doctors also consider the following:

  • What other drugs the person is taking because older people tend to take many drugs and drug interactions are a risk

  • Whether the antibiotic regimen is complex and hard to follow

  • Whether the person has family members or caregivers who can help the person take the antibiotic as prescribed

  • Whether the person lives in a nursing home because different bacteria may cause infections in such situations

Antibiotics need to be taken until the infecting bacteria are eliminated from the body, which may be days after the symptoms disappear. Stopping treatment too soon can result in a return of the infection.

A doctor, nurse, or pharmacist can explain how the prescribed antibiotic should be taken and what side effects it may have. Some antibiotics must be taken on an empty stomach. Others should be taken with food. Metronidazole Metronidazole and Tinidazole Metronidazole is an antibiotic used to treat pelvic, abdominal, soft-tissue, gum, and tooth infections and abscesses in the lungs or brain. It is also the preferred drug for certain protozoal... read more , a common antibiotic, causes an unpleasant reaction with alcohol. Also, some antibiotics can interact with other drugs that people may be taking, possibly reducing the effectiveness or increasing the side effects of the antibiotic or the other drugs. Some antibiotics make the skin sensitive to sunlight Chemical photosensitivity Photosensitivity, sometimes referred to as a sun allergy, is an immune system reaction that is triggered by sunlight. Sunlight can trigger immune system reactions. People develop itchy eruptions... read more Chemical photosensitivity .

Taking antibiotics to prevent infections

Antibiotics are sometimes used to prevent infections (called prophylaxis). For example, prophylactic antibiotics may be given to

  • People who have been exposed to a person with meningitis to prevent meningitis from developing

  • Some people with abnormal or artificial heart valves before dental and surgical procedures to prevent bacteria from infecting the damaged or artificial valves (such procedures can allow bacteria to enter the body)

  • People undergoing surgery that has a high risk of introducing an infection (such as major orthopedic or intestinal surgery)

Antibiotics may also be given to people who have a weakened immune system Infections in People With Impaired Defenses Many disorders, drugs, and other treatments can cause a breakdown in the body’s natural defenses. Such a breakdown can lead to infections, which can even be caused by microorganisms that normally... read more , such as people with leukemia, people taking chemotherapy for cancer, or people with AIDS, because such people are particularly susceptible to serious infections. They may need to take the antibiotics for a long time.

Taking antibiotics during pregnancy and breastfeeding

Most antibiotics pass into breast milk in large enough amounts to affect a breastfed baby and sometimes cannot be used in women who are breastfeeding. Sometimes a decision to stop breastfeeding or to not use the drug must be made.

Home Antibiotic Therapy

Usually, antibiotics taken outside the hospital are given by mouth. However, some infections—such as many of those involving bone (osteomyelitis Osteomyelitis Osteomyelitis is a bone infection usually caused by bacteria, mycobacteria, or fungi. Bacteria, mycobacteria, or fungi can infect bones by spreading through the bloodstream or, more often, by... read more ) or the heart (endocarditis Infective Endocarditis Infective endocarditis is an infection of the lining of the heart (endocardium) and usually also of the heart valves. Infective endocarditis occurs when bacteria enter the bloodstream and travel... read more Infective Endocarditis )—may require antibiotics to be given by vein (intravenously) for a long time, often 4 to 6 weeks. If people have no other conditions that need treatment in the hospital and are feeling relatively well, intravenous (IV) antibiotics may be given at home.

When antibiotics have to be given a long time, the short IV catheters that are inserted into a small vein in the arm or hand (such as those used in most routine hospital procedures) may not be desirable. These catheters last only up to 3 days. Instead, a special type of IV catheter is used. It may be inserted either

  • Directly into a large central vein, usually in the neck or chest (called a central catheter)

  • Into a small vein in the arm and threaded into a large central vein (called a peripherally inserted central catheter, or a PICC)

Some devices for giving antibiotics IV are simple enough that people and their family members can learn to operate them on their own. In other cases, a visiting nurse must come to the home to give each dose. In either situation, people are carefully supervised to make sure the antibiotic is being given correctly and to watch for possible complications and side effects.

If antibiotics are given at home through an IV catheter, the risk of developing an infection at the site where the catheter is inserted and in the bloodstream is increased. The following may indicate a catheter-related infection:

  • Pain, redness, and pus at the catheter insertion site

  • Chills and fever (even without problems at the insertion site)

Side Effects of Antibiotics

Common side effects of antibiotics include

Some side effects are more severe and, depending on the antibiotic, may impair the function of the kidneys, liver, bone marrow, or other organs. Blood tests are sometimes done to determine whether these organs have been affected.

Colitis, an inflammation of the large intestine (colon), develops in some people who take antibiotics, especially cephalosporins, clindamycin, fluoroquinolones, or penicillins. This type of colitis, called Clostridioides difficile–induced colitis Clostridioides (formerly Clostridium) difficile–Induced Colitis Clostridioides difficile (C. difficile)–induced colitis is an inflammation of the large intestine (colon) that results in diarrhea. The inflammation is caused by toxin produced... read more , results from toxins produced by the bacteria Clostridioides difficile (C. diff). These bacteria are resistant to many antibiotics and grow in the intestines unchecked when other normal bacteria in the intestine are killed by the antibiotics. Clostridioides difficile–induced colitis can be difficult to treat and can be life threatening, especially in older people.

Allergic Reactions to Antibiotics

It is important for people to tell health care practitioners if they are allergic to a particular antibiotic and to describe their past reaction when treated with that antibiotic. Many people have side effects when taking an antibiotic, but these effects may not be allergy-related (see Allergies to Drugs Allergies to Drugs People sometimes mistake many adverse drug reactions for allergies. For example, people who experience stomach discomfort after taking aspirin (a common adverse reaction) often say they are... read more ). The distinction is important because people who are allergic to an antibiotic should not be given that drug or an antibiotic closely related to it. That is because allergic reactions may be life threatening. However, people who have experienced only minor side effects can usually take related drugs or even continue taking the same drug. Health care practitioners can determine the significance of any unpleasant reaction people have to an antibiotic.

Drugs Mentioned In This Article

Generic Name Select Brand Names
FIRVANQ, Vancocin, Vancocin Powder, VANCOSOL
A/T/S, Akne-mycin, E.E.S., Emcin Clear , EMGEL, E-Mycin, ERYC, Erycette, Eryderm , Erygel, Erymax, EryPed, Ery-Tab, Erythra Derm , Erythrocin, Erythrocin Lactobionate, Erythrocin Stearate, Ilosone, Ilotycin, My-E, PCE, PCE Dispertab , Romycin, Staticin, T-Stat
Azasite, Zithromax, Zithromax Powder, Zithromax Single-Dose , Zithromax Tri-Pak, Zithromax Z-Pak, Zmax, Zmax Pediatric
Azactam, Cayston
Zyvox, Zyvox Powder, Zyvox Solution
Cleocin, Cleocin Ovules, Cleocin Pediatric, Cleocin T, CLIN, Clindacin ETZ, Clindacin-P, Clinda-Derm , Clindagel, ClindaMax, ClindaReach, Clindesse, Clindets, Evoclin, PledgaClin, XACIATO
Cubicin, Cubicin RF
Xenleta, Xenleta Solution
Flagyl, Flagyl ER, Flagyl RTU, LIKMEZ, MetroCream, MetroGel, MetroGel Vaginal, MetroLotion, Noritate, NUVESSA, Nydamax, Rosadan, Rozex, Vandazole, Vitazol
Bactroban, Centany, Centany AT
Furadantin, Macrobid, Macrodantin, Urotoin
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