Antibiotics

What HowWhy

What Antibiotics are drugs derived wholly or

partially from certain microorganisms Are used to treat bacterial or fungal

infections. They are ineffective against viruses 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, a doctor makes a best guess as to which bacterium is responsible.

For some infections, doctors know that only certain types of bacteria may be responsible.

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

For infections that may be caused by many different types of bacteria or by bacteria whose susceptibility to antibiotics is not predictable, a laboratory will be asked to identify the infecting bacterium from samples of blood, urine, or tissue taken from the person

The infecting bacterium is then tested for susceptibility to a variety of antibiotics. These tests generally take a day or two to yield results and thus cannot guide the initial choice of which antibiotic is given

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.

In selecting which antibiotic to use, a doctor also considers 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.

How

For severe bacterial infections, antibiotics are usually first given by injection. When the infection is under control, antibiotics can then be taken by mouth. Less severe infections can be treated from the start with oral antibiotics.

Taking Antibiotics

Antibiotics need to be taken until the infecting organism is 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).

A doctor, nurse, or pharmacist can explain how the prescribed antibiotic should be taken.

Some antibiotics must be taken on an empty stomach, whereas others may be taken with food.

Also, some antibiotics can interact with other drugs a person 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.

Prophylaxis In addition to treating existing infections,

antibiotics are sometimes used to prevent infections

Antibiotics may be used to prevent meningitis in people who have been exposed to someone with meningitis.

Some people with abnormal or artificial heart valves take antibiotics before dental and surgical procedures to prevent bacteria from infecting the damaged valves.

Prophylactic antibiotics may also be given to people who have a weakened immune system, such as people with leukemia, people receiving chemotherapy for cancer, or people with AIDS.

Side Effects and Allergies 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 disrupt the function of the kidneys, liver, bone marrow, or other organs.

Blood tests are used to monitor such adverse reactions.

Some people who receive antibiotics develop colitis, an inflammation of the large intestine.

The colitis results from a toxin produced by the bacterium Clostridium difficile, which grows unchecked when other antibacteria are killed by the antibiotics.

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 in fact they have only experienced side effects from it that are not allergy-related.

The distinction is important because people who are allergic to an antibiotic should not be given that drug or one closely related to it.

However, people who have experienced minor side effects can usually take related drugs or even continue taking the same one. The doctor can determine the significance of any unpleasant reaction a person has to an antibiotic

Antibiotic Resistance

Antibiotic resistance is a worldwide public health problem that continues to grow. It occurs when strains of bacteria in the human body become resistant to antibiotics due to improper use and abuse of antibiotics.

In hospitals, 190 million doses of antibiotics are administered each day.

Among non-hospitalized patients, more than 133 million courses of antibiotics are prescribed by doctors each year.

It is estimated that 50 percent of these latter prescriptions are unnecessary since they are being prescribed for colds, coughs and other viral infections.

Common causes

Many individuals either expect or ask their physicians to prescribe antibiotics when they feel sick or have a common cold.

Patients should understand, though, that antibiotics are intended to treat bacterial infections, not viral infections.

The only true way to know if your cold or sickness is a bacterial infection and whether it should be treated with antibiotics is for your physician to test it.

If you have a sore throat your physician should take a throat culture test. If the the test results indicate that a bacterial infection is present, then antibiotics should be prescribed to treat the infection.

The improper use and abuse of antibiotics has led to the development of antibiotic resistance. The most common misuse and abuse of antibiotics are: Physicians prescribing antibiotics for viral

infections Not finishing the full dosage of the antibiotic.

When an antibiotic prescription is not finished (even leaving one or two pills), it leaves some bacteria alive and "resistant" to future antibiotic treatment.

Prevention Both physicians and patients have a role to

play in decreasing the misuse of antibiotics. Antibiotics should only be prescribed when a test (such as a throat culture) shows that there is a bacterial infection present. Antibiotics are not effective in fighting a viral infection. Even so, patients often demand that their physicians prescribe antibiotics when they are not needed. Taking antibiotics when you have a viral infection not only wastes your time and money, but also contributes to increasing antibiotic resistance.

What Can You Do

Patients should ask their doctor if they have a viral or bacterial infection and which tests have been done to prove this. Physicians too, must change their prescribing practices and only prescribe antibiotics for their patients when a bacterial infection is present

Doctors, pharmacists, and researchers knew that some medicines caused serious side effects or reacted dangerously with other drugs. But there were few tests to predict these problems before drugs went into clinical studies or were used by many people.

Copyright © 1995-2006 Merck & Co., Inc., Whitehouse Station, NJ, USA. .

Understanding How Drugs Work In the Body

Thirty Years Ago Most studies on drug-metabolizing enzymes

were done in rats or mice, so scientists didn’t know much about these enzymes in humans.

To infer if a drug candidate would be toxic to humans, scientists evaluated whether the molecule caused organ damage in animals.

Drug makers lacked the ability to customize the packaging and delivery of a drug to ensure that it would be well absorbed and available to the body.

Today

As recently as 10 to 15 years ago, up to 40 percent of drugs failed to work properly because they were poorly absorbed, were destroyed by the body, failed to get to the right place, or were excreted from the body too quickly.

Fewer than 10 percent of medicines fail for these reasons. In part, that’s because scientists are able to identify which enzymes metabolize a candidate drug and what the end products will be. The Food and Drug Administration now requires this information before it considers approving a new drug.

Researchers have characterized dozens of human drug-metabolizing enzymes and transport proteins that regulate the activity and levels of drugs in the body.

Scientists also have identified certain medicines, vitamins, herbal remedies, nutritional supplements, and other compounds that interact with these enzymes and transporters, possibly causing adverse cross-reactions. To minimize dangerous interactions, doctors and pharmacists maintain lists of such substances. Pharmaceutical scientists are able to detect potentially troublesome compounds early in drug discovery so they can prevent these compounds from moving forward in development.

By analyzing the genetic sequences of drug-metabolizing enzymes from many people, researchers have identified more than 100 slightly different versions of the enzymes. Although most of these genetic variations are rare, some of them can markedly alter the activity and side effects of drugs.

As scientists learn more about drug-metabolizing enzymes, particularly those called P450s, they are able to design and develop drugs that influence the activity of the enzymes.

Scientists are now able to use human, rather than animal, enzymes to predict whether a drug candidate or any of its byproducts will be toxic to humans. However, rare, serious drug reactions remain difficult to predict before testing experimental medicines in humans.

Patient instructions routinely indicate whether a drug should be taken at a particular time of day and whether oral medications should be consumed with a meal or on an empty stomach.

National Institute of General Medical Sciences