Overview of Antibiotics

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:

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:

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

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 .

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

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:

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.

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.