Antibiotics are medications used to treat bacterial infections. They are not effective against viral infections and most other infections. Antibiotics either kill bacteria or stop them from reproducing, allowing the body's natural defenses to eliminate them.
Doctors try to use antibiotics for specific bacterial infections, but they sometimes start antibiotics that can treat many different bacteria while waiting for results of tests that identify the specific bacteria.
Taking the antibiotics as prescribed is important, and antibiotics should be taken in the dose, frequency, and number of days that are most effective to treat a specific infection.
Bacteria can develop resistance to the effects of antibiotics, especially if they are not taken as directed.
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:
Streptogramins (such as quinupristin and dalfopristin)
Carbapenems, cephalosporins, monobactams, and penicillins are subclasses of beta-lactam antibiotics. Beta-lactam antibiotics are 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, , , , , , , , and .
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 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 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
How well the drug is absorbed into the bloodstream (for antibiotics taken by mouth)
How much of the drug reaches the sites of infection in the body (see Drug Distribution)
How quickly the body eliminates the drug (see Drug Elimination)
These factors may vary from person to person, depending on other medications 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 antibiotic fight the infection)
The antibiotic’s possible side effects
The possibility of allergies or other serious reactions to the antibiotic
The cost of the antibiotic
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 antibiotic 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 1 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 (for example, when antibiotics are not taken as prescribed), bacteria are frequently exposed to these medications. Although many bacteria die when exposed to antibiotics, if antibiotics are not taken appropriately, some bacteria survive and develop resistance to the antibiotics’ effects. For example, in the past, 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 penicillin ineffective. Researchers responded by developing a form of penicillin that the enzyme could not break down, but after a few years, the bacteria adapted again and became resistant to this modified penicillin. Other bacteria have also developed resistance to antibiotics.
Medical research continues to develop antibiotics 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...
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Taking Antibiotics
For severe bacterial infections or for people who cannot keep down food or liquids, 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.
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. , a common antibiotic, causes an unpleasant reaction with alcohol. Also, some antibiotics can interact with other medications that people may be taking, possibly reducing the effectiveness or increasing the side effects of the antibiotic or the other medications. 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
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 prophylactic 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 HIV/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 are among the safest antibiotics to use during pregnancy. (See also Safety of Medications 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 antibiotic 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 Medication and Substance Use During Breastfeeding.)
Home Antibiotic Therapy
Usually, antibiotics taken outside the hospital are given by mouth. 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 for 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
Upset stomach
In women, vaginal yeast infections
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,Clostridioides difficile–induced colitis, 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 adults.
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.
It is important for people to tell health care professionals 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). 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 antibiotics or even continue taking the same antibiotic. Health care professionals can determine the significance of any unpleasant reaction people have to an antibiotic.
