MSD Manual

Please confirm that you are not located inside the Russian Federation

honeypot link

Overview of Antibiotics


Brian J. Werth

, PharmD, University of Washington School of Pharmacy

Last full review/revision Jul 2020| Content last modified Jul 2020
Click here for the Professional Version
Topic Resources

Antibiotics are drugs used to treat bacterial infections. They are ineffective against viral infections and most other infections. Antibiotics either kill microorganisms or stop them from reproducing, allowing the body's natural defenses to eliminate them.

  • Although doctors try to use antibiotics for specific bacterial infections, they sometimes start antibiotics without waiting for tests that identify the specific bacteria.

  • Bacteria can develop resistance to the effects of antibiotics.

  • Antibiotics can have side effects, such as upset stomach, diarrhea, and, in women, vaginal yeast infections.

  • Some people are allergic to certain antibiotics.

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, clindamycin, daptomycin, fosfomycin, lefamulin, metronidazole, mupirocin, nitrofurantoin, and tigecycline.

Selecting an Antibiotic

Each antibiotic is effective only against certain bacteria. In selecting an antibiotic to treat a person with an infection, doctors estimate 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 from samples of blood, urine, or tissue taken from the person (see Diagnosis of Infectious Disease). 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. 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, bacteria are constantly exposed to these drugs. Although many bacteria die when exposed to antibiotics, some develop resistance 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

  • Taking antibiotics only when necessary (that is, people should take antibiotics only for infections caused by bacteria, not for those caused by viruses such as a cold or the flu)

  • Not asking doctors to prescribe antibiotics for viral infections, such as the common cold or the flu

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 severe bacterial infections, antibiotics are usually first given by injection (usually into a vein but sometimes into a muscle). When the infection is controlled, antibiotics can then be taken by mouth.

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.)

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. Antibiotics are rarely given for fewer than 5 days. (An exception is certain uncomplicated urinary tract infections.) 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, 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.

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)

To avoid the development of antibiotic resistance in bacteria and side effects in people, doctors usually give preventive antibiotics for only a short time.

Antibiotics may also be given to people who have a weakened immune system, 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

Generally, antibiotics are used during pregnancy only when the benefits of treatment outweigh the risks. Some antibiotics are safer than others. Penicillins, cephalosporins, and erythromycin are among the safest antibiotics to use during pregnancy. Tetracyclines are not used during pregnancy. (See also Drug Use During Pregnancy.)

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.

If an infection develops during pregnancy or while breastfeeding, women should talk to their doctor about the benefits and risks of treatment. (See also Drug Use During Breastfeeding.)

Home Antibiotic Therapy

Usually, antibiotics are given by mouth, and the length of treatment does not cause hardship. However, some infections—such as many of those involving bone (osteomyelitis) or the heart (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, results from toxins produced by the bacteria Clostridioides difficile. 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

Antibiotics can also cause allergic reactions. Mild allergic reactions may consist of an itchy rash or slight wheezing. Severe allergic reactions (anaphylaxis) can be life threatening and usually include swelling of the throat, inability to breathe, and low blood pressure.

Many people tell their doctor that they are allergic to an antibiotic when they have experienced only side effects that are not allergy-related (see Allergies to Drugs). 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. 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.

NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
Others also read
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID

Test your knowledge

Respiratory Syncytial Virus (RSV) Infection and Human Metapneumovirus Infection
Respiratory syncytial virus (RSV) infection and human metapneumovirus infection (hMVP) cause upper and sometimes lower respiratory tract infections. Symptoms of both viruses are similar although the viruses are different. In which of the following ages groups is RSV a very common cause of respiratory tract infections?
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID

Also of Interest

Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID