Antibiotics are drugs used to treat bacterial infections Bacterial Infections 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 . 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 Defenses Against Infection Natural barriers and the immune system defend the body against organisms that can cause infection. (See also Lines of Defense.) Natural barriers include the skin, mucous membranes, tears, earwax... read more 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 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 .
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:
Glycopeptides and lipoglycopeptides Glycopeptides and Lipoglycopeptides Glycopeptides and lipoglycopeptides are antibiotics used to treat complicated and/or serious infections caused by gram-positive bacteria. Glycopeptides and lipoglycopeptides include the following... read more (such as vancomycin)
Oxazolidinones Oxazolidinones: Linezolid and Tedizolid Oxazolidinones are a class of antibiotics used to treat serious infections, often after other antibiotics have been ineffective. Oxazolidinones include the following: Linezolid Tedizolid Oxazolidinones... read more (such as linezolid and tedizolid)
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
How much of the drug reaches the sites of infection in the body (see Drug Distribution Drug Distribution Drug distribution refers to the movement of a drug to and from the blood and various tissues of the body (for example, fat, muscle, and brain tissue) and the relative proportions of drug in... read more )
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
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...
For severe bacterial infections or for people who cannot keep down food or liquids, antibiotics are usually first given by injection Oral route Drugs are introduced into the body by several routes. They may be Taken by mouth (orally) Given by injection into a vein (intravenously, IV), into a muscle (intramuscularly, IM), into the space... read more (usually into a vein but sometimes into a muscle). When the infection is controlled, antibiotics can then be taken by mouth Oral route Drugs are introduced into the body by several routes. They may be Taken by mouth (orally) Given by injection into a vein (intravenously, IV), into a muscle (intramuscularly, IM), into the space... read more .
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. 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 .
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 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 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 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
Generally, antibiotics are used during pregnancy only when the benefits of treatment outweigh the risks. Some antibiotics are safer than others. Penicillins Penicillins Penicillins are a subclass of antibiotics called beta-lactam antibiotics (antibiotics that have a chemical structure called a beta-lactam ring). Carbapenems, cephalosporins, and monobactams... read more , cephalosporins Cephalosporins Cephalosporins are a subclass of antibiotics called beta-lactam antibiotics (antibiotics that have a chemical structure called a beta-lactam ring). Beta-lactam antibiotics also include carbapenems... read more , and erythromycin Macrolides Macrolides are a class of antibiotics that are often used to treat infections in people who are allergic to penicillins. Macrolides include the following: Azithromycin Clarithromycin Erythromycin read more are among the safest antibiotics to use during pregnancy. Tetracyclines Tetracyclines Tetracyclines are a group of antibiotics used to treat many different bacterial infections. Tetracyclines include the following: Doxycycline Eravacycline Minocycline read more are not used during pregnancy. (See also Drug Use During Pregnancy Drug Use During Pregnancy More than 50% of pregnant women take prescription or nonprescription (over-the-counter) drugs or use social drugs (such as tobacco and alcohol) or illicit drugs at some time during pregnancy... read more .)
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 Drug Use During Breastfeeding When mothers who are breastfeeding have to take a drug, they wonder whether they should stop breastfeeding. The answer depends on the following: How much of the drug passes into the milk Whether... read more .)
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 )—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
Antibiotics can also cause allergic reactions 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 . Mild allergic reactions may consist of an itchy rash or slight wheezing. Severe allergic reactions ( anaphylaxis Anaphylactic Reactions Anaphylactic reactions are sudden, widespread, potentially severe and life-threatening allergic reactions. Anaphylactic reactions often begin with a feeling of uneasiness, followed by tingling... read more ) 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 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|
|No US brand name|
|FURADANTIN, MACROBID, MACRODANTIN|