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Antibiotics -bī-ˈät-ik; -bē-

By Matthew E. Levison, MD, Adjunct Professor of Medicine;Professor School of Public Health, Drexel University College of Medicine;Drexel University

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  • 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.

  • Taking antibiotics as directed, even after symptoms disappear, is essential to curing the infection and to preventing the development of resistance in bacteria.

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

  • Some people are allergic to certain antibiotics.

Antibiotics (antibacterials) are drugs derived wholly or partially from bacteria or molds and are used to treat bacterial infections. They are ineffective against viral infections and fungal infections. Antibiotics either kill microorganisms or stop them from reproducing, allowing the body's natural defenses to eliminate them.

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. If one antibiotic is predictably effective against all of these bacteria, further testing is not 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.

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, how much of the drug reaches the sites of infection in the body, and how quickly the body eliminates the drug. 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 nature and seriousness of the infection, the drug’s possible side effects, the possibility of allergies or other serious reactions to the drug, and the cost of the drug.

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



Common Uses

Some Side Effects









Infections caused by gram-negative bacteria, such as Escherichia coli and Klebsiella species

Hearing loss


Kidney damage






Gangrene, sepsis, pneumonia, abdominal and urinary infections,

infections due to susceptible bacteria resistant to other antibiotics, and (except for ertapenem) Pseudomonas infections

Seizures (especially with imipenem)


Cephalosporins, 1st generation




Mainly skin and soft-tissue infections

Gastrointestinal upset and diarrhea


Allergic reactions

Cephalosporins, 2nd generation






Some respiratory and, for cefoxitin, abdominal infections

Gastrointestinal upset and diarrhea


Allergic reactions

Cephalosporins, 3rd generation











Given by mouth: Broad coverage of many bacteria for people with mild-to-moderate infections, including skin and soft-tissue infections

Given by injection: Serious infections (such as meningitis or infections acquired in a hospital)

Gastrointestinal upset and diarrhea


Allergic reactions

Cephalosporins, 4th generation


Serious infections (including Pseudomonas infections), particularly in people with a weakened immune system and infections due to susceptible bacteria that are resistant to other antibiotics

Gastrointestinal upset and diarrhea


Allergic reactions

Cephalosporins, 5th generation


Complicated skin infections (including foot infections in people with diabetes) due to susceptible bacteria, such as Escherichia coli, Pseudomonas aeruginosa, and methicillin-resistant Staphylococcus aureus (MRSA)









Sepsis, urinary tract infections, bacterial prostatitis, bacterial diarrhea, and gonorrhea

Nausea (rare)

Nervousness, tremors, and seizures

Numbness and tingling in the hands and feet (peripheral neuropathy)

Inflammation or rupture of tendons

Abnormal heart rhythms (arrhythmias)

Antibiotic-associated diarrhea and inflammation of the colon (colitis)

With trovafloxacin, sometimes fatal liver damage



Complicated abdominal infections and complicated skin infections due to susceptible bacteria, such as Escherichia coli, Staphylococcus aureus (including those resistant to methicillin), and anaerobes

Gastrointestinal upset

Sensitivity to sunlight

Permanent staining of teeth in the fetus if used late in pregnancy or in children under 8 years of age







Streptococcal infections, syphilis, respiratory infections, mycoplasmal infections, and Lyme disease

Nausea, vomiting, and diarrhea (especially at higher doses)


Abnormal heart rhythms



Infections caused by gram-negative bacteria

Allergic reactions

Can be used in patients allergic to antibiotics such as penicillins, cephalosporins, and carbapenems









Penicillin G

Penicillin V



Wide range of infections, including streptococcal infections, syphilis, and Lyme disease

Nausea, vomiting, and diarrhea

Allergy with serious anaphylactic reactions

Brain and kidney damage (rare)




Polymyxin B

Ear, eye, skin, or bladder infections

Usually applied directly to the skin, and rarely given by injection

Kidney and nerve damage (when given by injection)








Urinary tract infections (except sulfasalazine, sulfacetamide, and mafenide)

For mafenide, only topically for burns

Nausea, vomiting, and diarrhea

Allergy (including skin rashes)

Crystals in urine (rare)

Decrease in white blood cell and platelet counts

Sensitivity to sunlight

Possibly increased tendency to bleed if used with warfarin







Syphilis, chlamydial infections, Lyme disease, mycoplasmal infections, and rickettsial infections

Gastrointestinal upset

Sensitivity to sunlight

Staining of teeth in the fetus if used late in pregnancy or in children under 8 years of age

Miscellaneous antibiotics


Typhoid, other salmonellal infections, and meningitis

Severe decrease in white blood cell count (rare)


Streptococcal and staphylococcal infections, respiratory infections, and lung abscess

Antibiotic-associated diarrhea and inflammation of the colon (colitis)


Complicated skin infections, bloodstream infections, and certain heart valve infections (endocarditis) due to susceptible bacteria, including methicillin-resistant Staphylococcus aureus (MRSA)

Not used when infection involves the lungs

Gastrointestinal upset

Muscle pain and weakness



Vision disturbances


Bladder infections




Nausea and vomiting



Serious infections caused by gram-positive bacteria that are resistant to many other antibiotics




Anemia and low white blood cell and platelet counts

Numbness and tingling in the hands and feet (peripheral neuropathy)

Visual disturbances

Confusion, agitation, tremors or coma in some people who also use selective serotonin-reuptake inhibitors (SSRIs)


Vaginitis caused by Trichomonas or Gardnerella species and pelvic and abdominal infections


Headache (especially if the drug is taken with alcohol)

Metallic taste

Numbness and tingling in the hands and feet (peripheral neuropathy)

Dark urine


Urinary tract infections

Nausea and vomiting




Liver dysfunction

Gout (occasionally)


Serious infections caused by gram-positive bacteria that are resistant to other antibiotics

Aching muscles and joints


Tuberculosis and leprosy


Liver dysfunction

Red-orange saliva, sweat, tears, and urine






Mild to moderate community-acquired pneumonia

Visual disturbances

Liver damage (possibly fatal)

Worsening of symptoms in people with myasthenia gravis (possibly fatal)


Serious infections, especially those due to MRSA, Enterococcus, or bacteria resistant to other antibiotics

Flushing, itching

Allergic reactions

Decrease in white blood cell and platelet counts

*Polypeptide antibiotics are usually applied directly to the skin or eye and are rarely given by injection.

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 (see Antibiotic resistance). 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 patients and doctors can help prevent the development of resistance in bacteria. Taking antibiotics only when necessary can help. 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. Also, taking antibiotics for the complete time prescribed helps limit the development of resistance.

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 be given 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. So people must take antibiotics for the entire time prescribed whether they have symptoms or not. 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 or the development of antibiotic-resistant bacteria.

Did You Know...

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

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 may be taken with food. Metronidazole, a common antibiotic, causes an unpleasant reaction with alcohol. Also, some antibiotics can interact with other drugs 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.

Antibiotics are sometimes used to prevent infections (called prophylaxis). 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 take antibiotics 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 with a high risk of introducing infection (such as major orthopedic or intestinal surgery) may be given antibiotics immediately before the operation. To be effective and to avoid the development of resistance in bacteria, doctors 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.

Home Antibiotic Therapy

Usually, antibiotics are given by mouth, and the length of treatment does not cause hardship. However, some infections—such as those involving bone (osteomyelitis) or the heart (endocarditis)—require antibiotics to be given 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 may be inserted into a large central vein, usually in the neck or chest.

Some devices for infusing antibiotics 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. Pain, redness, and pus at the catheter insertion site or chills and fever (even without problems at the insertion site) may indicate a catheter-related infection.

Side Effects and Allergic Reactions

Common side effects of antibiotics include upset stomach, diarrhea, and, 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 used to check for effects on kidney and other organ function.

Some people who take antibiotics, especially cephalosporins, clindamycin, or fluoroquinolones, develop colitis, an inflammation of the large intestine. This type of colitis results from toxins produced by the bacteria Clostridium difficile, which is resistant to many antibiotics and which grows in the intestines unchecked when other normal bacteria in the intestine are killed by the antibiotics (see Clostridium difficile -Induced Colitis).

Antibiotics can also cause allergic reactions. Mild allergic reactions 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 only experienced side effects that are not allergy-related. 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. Doctors can determine the significance of any unpleasant reaction people have to an antibiotic.

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