What Are Antibiotics?

Ruth Jessen Hickman, MD
·11 min read
<p>Israel Sebastian / Getty Images</p>

Israel Sebastian / Getty Images

Medically reviewed by Lindsay Cook, PharmD

Antibiotics are an important class of drugs used to fight infections caused by bacteria. Medical professionals use them to treat a wide range of infections, such as pneumonia, strep throat, meningitis, urinary tract infections, and skin infections, among many others.

However, not all infections are caused by bacteria. Antibiotics won’t be effective in treating infections caused by viruses or other non-bacterial infectious organisms.

Most antibiotics need to be prescribed by a healthcare provider, but some antibiotics applied to the skin are available over-the-counter.

What Do Antibiotics Do?

Simply put, antibiotics are drugs that kill bacteria. They are used to treat bacterial infections of many different types. In a few cases, they might also be used to help prevent future infections, such as during an operation or in some kinds of dental care.

Different classes of antibiotics kill bacteria using different mechanisms. For example, antibiotics in the penicillin group block the formation of a structure normally found in some bacteria (the cellular wall). Another example is Cipro XR (ciprofloxacin), which kills bacteria by preventing them from making DNA.

Some people need to take antibiotics for less serious infections that can be treated at home. Antibiotics are also an important part of treatment for many hospitalized people who may be more seriously ill from an infection.

Antibiotics start to work fairly quickly, but it may take a day or two before you start to see improvements in your symptoms.

Examples of the many important applications for antibiotics include the following:

However, antibiotics aren’t helpful for infections caused by other infectious organisms. For example, they are unlikely to be helpful for the following:

Many ear infections and sinus infections don’t require antibiotics, but some do.

Types of Antibiotics

Although all antibiotics work to kill bacteria, not all antibiotics are the same. Many antibiotics are effective against certain types of bacteria but not others. So, for a specific type of infection, a healthcare provider might need to prescribe you an antibiotic of a certain class.

Classes of antibiotics are also important because some side effects or allergies may apply to all or most of the members of a class.

Some of the most common classes of antibiotics are briefly described below. However, even within these classes, health professionals often distinguish between subsets, which may have differences in the specific bacteria they can successfully treat.

Beta-lactam Antibiotics

Included in this group are penicillins. They were the first antibiotics developed, and they still are one of the most commonly used around the globe. Examples are Bicillin-LA (penicillin g benzathine), Ampi (ampicillin), Amoxil (amoxicillin).

A closely related class are the cephalosporins, which includes many different antibiotics treating a wide range of infections. The generic names of these drugs usually start with “ceph” or “cef.” Some important examples are Kefzol (cefazolin), Keflex (cephalexin), Mefoxin (cefoxitin), and Rocephin (ceftriaxone). 

Carbapenems are somewhat structurally related to cephalosporins and penicillins. They tend to be used in very ill people who might be resistant to other antibiotics. An important example is Primaxin (imipenem packaged with cilastatin, a drug which prevents its degradation).

Beta-lactam antibiotics are sometimes packaged with another type of drug, called beta lactamase inhibitors, that tends to make them more effective. Important examples are Augmentin (amoxicillin/clavulanate) and Zosyn (piperacillin/tazobactam).

Aminoglycosides

Aminoglycosides are an old class of antibiotics that target a wide range of bacteria. They are still sometimes a useful one, but they do carry some toxicities. They include Gentak (gentamicin), neomycin, and streptomycin, among others.

Sulfonamides

Drugs in the class, often called "sulfa", are usually combined with specific other antibiotic agents to make them more effective. For example, Bactrim is sulfamethoxazole combined with trimethoprim. This very affordable drug can treat a wide variety of bacterial infections.

Quinolones

Quinolones generally have “floxacin” in their names. They can fight a broad spectrum of bacteria, but other agents are often preferred, if available, because they can sometimes have severe side effects. Some examples are Cipro XR (ciprofloxacin), Levaquin (levofloxacin), and Avilox (moxifloxacin).

Macrolides

Macrolides generally have “mycin” at the end of their generic name. They are older antibiotics which can be used to treat a wide variety of types of infections. Zithromax (azithromycin), Klaricid (clarithromycin), and Erythrocin (erythromycin) are some of the antibiotics in this class.

Nitroimidazole antibiotics

Some medications in this class work against some bacteria but also against some other organisms such as protozoa. Flagyl (metronidazole) is one of the best known medications in this class.

Tetracyclines

Tetracyclines are a class of antibiotics that treat a broad class of infections, some of which are a little more unusual (e.g. Legionnaires’ disease or Lyme disease). Some key examples are Panmycin (tetracycline), Vibramycin-D (doxycycline), and Dynacin (minocycline).

Lincosamides

The most used medication in this class is Cleocin (clindamycin). It can be used to treat some very serious infections, including sepsis. It is also sometimes used to help prevent future infections, e.g., during surgery.

Oxazolidinones and Lipoglycopeptides

These are two of the newest classes of antibiotics. They may be used to treat some dangerous skin infections that are otherwise resistant to antibiotics. Zyvox (linezolid) is an important oxazolidinone, and Dalvance (dalbavancin) is an example of a lipoglycopeptide.

Glycopeptides

These antibiotics may be helpful in very serious infections, but they carry certain risks of toxicity. An important example is Firbanq (vancomycin).

How Are Antibiotics Given?

Antibiotics are packaged in many different ways. Most commonly, you take the antibiotic as a pill to swallow. Antibiotics for children are also often given via liquid medicine to drink. Usually, you take the antibiotic over the course of several days; oral antibiotics are generally given in a five, seven, or ten-day course.

Depending on the situation, different modes of delivery might be needed. For an infection on your skin, you might apply an antibiotic cream directly to the area. Or if you had an infection of your eye, you might get a prescription for an antibiotic eye drop.

In some cases, it’s important to get the antibiotic more deeply into the body more quickly. For example, for an internal infection in a specific location, you might need an antibiotic injection to get the medicine more directly to the area.

Intravenous (IV) antibiotics when the antibiotic needs to be spread around the body quickly. They might also be needed for some types of infections for which oral antibiotics don’t tend to work as well, like a joint infection.

Potential Side Effects

Antibiotics have a long list of potential side effects. Some are specific to a type of antibiotics, like a risk of hearing loss for aminoglycosides. Others tend to be side effects of many kinds of antibiotics.

Some of the most common side effects from antibiotics are:

Side effects are usually mild and manageable. However, more serious side effects are also possible. It’s very rare, but some people might experience a very severe and even life-threatening allergic reaction to an antibiotic.

Or you might experience liver or kidney damage, or a serious infection if you receive the drug through an intravenous line. You also might get an antibiotic-resistant infection that could be hard to treat. Some antibiotics can put you at risk of a bacterial infection called C difficile (C. diff), which can be life-threatening in some people.

Some antibiotics have greater potential for severe side effects. Your healthcare provider is unlikely to prescribe them unless there aren’t better options available for treating you.

Other Considerations

There are many factors to consider when taking antibiotics.

Antibiotic Overuse

Antibiotics are powerful and sometimes life-saving drugs. However, it’s also important that they not be used unless they are really needed. For one thing, antibiotics have side effects, which can sometimes be significant. So, it makes sense not to use them if they aren’t necessary.

Another aspect is the impact on the community. The more that healthcare providers prescribe antibiotics, the more bacteria develop that are resistant to those antibiotics, meaning the antibiotics simply no longer work to kill those bacteria.

This can lead to infections that are very hard to treat. Researchers estimate that 23,000 deaths in the United States each year are due to infections from resistant bacteria.

So, if you have an infection that probably isn’t caused by bacteria, or if you have the sort of bacterial infection that you should easily fight off, an antibiotic may not be appropriate.

Taking Antibiotics as Prescribed

Relatedly, you should always take antibiotics exactly as prescribed. If you are feeling better and decide to stop taking the full course of your antibiotic, the infection might not be fully treated. You also increase the risk of bacterial resistance in your community.

You should also never take someone else’s unfinished antibiotic prescription, or an old one you never finished yourself. Because not all infections are bacterial, and not every antibiotic is effective against all bacteria, it’s unlikely to be helpful anyway.

In some cases, you’ll need to follow additional instructions to make sure your antibiotic is safe and effective. For example, you shouldn’t drink alcohol while you are taking certain kinds of antibiotics.

Underlying Conditions

Your medical provider will prescribe you an antibiotic based on the type of infection, but also based on your underlying medical conditions. For example, if you have underlying kidney or liver problems, you might need to avoid certain antibiotics, or you might need to take a different dose.

Also make sure to tell your healthcare provider about allergies you’ve had to antibiotics. For example, if you had a serious allergic reaction to a penicillin, it may not be safe for you to take penicillins or certain kinds of cephalosporins.

When to Call Your Healthcare Provider

Whenever you are taking an antibiotic, you’ll want to learn about potential side effects from your healthcare provider or pharmacist. That way you’ll know what mild side effects you might expect and which more serious side effects you should look out for.

Call 911 if you experience potentially life-threatening side effects such as difficulty breathing or sudden severe chest pain. If you have severe or unexplained side effects, like bloody diarrhea, significant abdominal pain, muscle weakness, or uncontrolled vomiting, stop taking the antibiotic and call your healthcare provider.

A Quick Review

Antibiotics are medications that fight bacterial infections. Medical professionals use them to treat a wide range of infections, such as pneumonia, strep throat, meningitis, urinary tract infections, and skin infections, among many others.

Antibiotics have different classes, which are effective for different types of bacteria. Based on the type of infection you have and your underlying medical problems, your healthcare provider will prescribe an antibiotic that is safe and effective for you. However, they often have side effects.

Antibiotics aren’t effective against all kinds of infections though. For viral and some other types of infections, they can do more harm than good.

Frequently Asked Questions

How can you tell if an antibiotic is working?

If you are taking an antibiotic for an infection, your symptoms should start to improve within a few days or so. If that isn't the case, get in contact with your prescribing healthcare provider. It's possible that you might need a different antibiotic or another kind of medical care.

Can an infection come back after using antibiotics?

That depends. You should receive a sufficient prescription for antibiotics to kill the bacteria off completely. However, in some cases you might be prescribed an antibiotic that is ineffective at killing the specific kind of infection you have. It's also possible to be cured of an infection in terms of your symptoms, e.g., strep throat, but be reinfected at a later date.

What's the difference between antibiotic and antiviral medication?

Antibiotics kill bacteria. Antivirals are another class of medications that fight viruses, a totally different kind of infectious microorganism. So antibiotics are appropriate for some kinds of infections, and antivirals are appropriate for others.

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