Laura Dorwart is a health journalist with expertise in disability rights, mental health, and pregnancy-related conditions. She has written for publications like SELF, The New York Times, VICE, and The Guardian.

Lindsay Cook, PharmD is a board-certified consultant pharmacist.

Antibiotics are medicines that are used to treat certain kinds of bacterial infections. They work to clear up infections either by killing bacteria or stopping their growth. They typically have to be prescribed by a healthcare provider.

Learn more about using antibiotics, including when and how to take them safely.

Antibiotics came into broader use in the 1940s, after British scientist Alexander Fleming discovered penicillin (the first antibiotic) in mold. Before Fleming started experimenting with penicillin, many people died of common bacterial infections.  

Today, antibiotics are safe and effective for most people to use. They are usually prescribed to treat common bacterial infections, such as strep throat and urinary tract infections (UTIs).

Antibiotics are divided into several classes based on how they work and what kinds of conditions they treat. The classes of antibiotics include:

Antibiotics can be taken in several different ways, including:

Usually, injectable and IV antibiotics are used for more serious infections that have started to spread.

It’s not necessary to take antibiotics any time you have an illness. Antibiotics are most effective in treating certain bacterial infections, such as:

Some infections may be caused either by bacteria or by a virus. Your healthcare provider will be able to determine whether you have a viral or bacterial infection and prescribe antibiotics (or not) accordingly. They may also prescribe antibiotics if your infection seems to be getting worse or spreading.

It may be tempting to take antibiotics whenever you’re sick. But antibiotics won’t treat conditions caused by viruses, such as:  

Viral infections often clear up without medical intervention, or they require antiviral medications or other forms of treatment. Your healthcare provider can help you decide what treatment will be appropriate and effective for your infection. 

Also, antibiotics only work to fight certain bacterial infections. They aren’t effective in treating all conditions caused by bacteria, including some ear infections and most sinus infections.

Antibiotic-resistant infections may occur when antibiotics aren’t able to kill off certain bacteria. Antibiotic resistance can cause serious health complications, such as organ failure. In some cases, it may even be fatal. There are over 2.8 million antibiotic-resistant infections in the U.S. each year, resulting in over 35,000 deaths.

You may be more likely to develop an antibiotic-resistant infection if you take antibiotics too often or unnecessarily. Only take antibiotics when you need them. Always finish taking your full course as prescribed (don't stop when you feel better, continue the number of days on the prescription).

While antibiotics are safe and effective for most people, they may sometimes cause side effects. Here are some ways to ensure that you’re taking antibiotics safely:

Some people develop allergy symptoms after taking antibiotics. Allergic reactions to penicillins are particularly common—an estimated 5% to 15% of people have a penicillin allergy.  

You may be experiencing an allergic reaction to an antibiotic if you have any of the following symptoms:

If you start experiencing any of these symptoms, stop taking the antibiotic right away and tell your healthcare provider.

If you experience symptoms of anaphylaxis (a severe allergic reaction) after taking antibiotics, such as difficulty breathing or swollen tongue or throat, dizziness or fainting, seek immediate medical help.

Outside of allergy symptoms, other common side effects of antibiotics may include:

In rare cases, antibiotics may cause more serious side effects, including:

If you experience any of these symptoms, seek medical help right away.

Most antibiotics are safe to take during pregnancy. However, some antibiotics—such as fluoroquinolones and tetracyclines—should be avoided if you’re currently pregnant. Let your healthcare provider know if you are pregnant or might be pregnant before you start taking an antibiotic.

Most of the time, antibiotics need to be prescribed by a healthcare provider. However, some over-the-counter (OTC) topical antibiotic treatments are available at pharmacies and drugstores.

Neosporin (neomycin, polymyxin, and bacitracin topical) is an OTC antibiotic ointment used to prevent wounds, cuts, and scrapes from getting infected.

In some cases, Neosporin can cause an allergic skin reaction. Seek medical help immediately if you notice any of the following symptoms:

Neosporin is only effective in treating minor skin injuries. Talk to your healthcare provider instead if you have a burn, deep wound, animal bite, or another more serious injury.

Antibiotics are medicines that treat certain bacterial infections, either by killing bacteria or by preventing their growth. Examples of common antibiotics include penicillin, azithromycin, clindamycin, and cephalexin.

Some of the conditions treated by antibiotics include strep throat, urinary tract infections (UTIs), and some types of pneumonia and sexually transmitted infections (STIs). Antibiotics can’t treat infections caused by viruses, including colds, flu, and runny noses. 

It’s important to take antibiotics exactly as your healthcare provider prescribes. If you take antibiotics too often, you can develop antibiotic-resistant infections.

Common side effects of antibiotics include diarrhea, stomach pain, nausea, vomiting, headache, yeast infections, and black or hairy tongue. If you develop allergy symptoms such as rash, hives, swelling, or difficulty breathing after taking antibiotics, seek medical help right away.

If you have a bacterial infection, antibiotics can help you feel better and avoid potentially life-threatening complications. However, it’s important to take antibiotics only when you need them and exactly as prescribed.

Antibiotics usually start working to clear up a bacterial infection right away. You may start feeling better in just one to three days. However, it’s important to take your full course of antibiotics, even if your symptoms have already subsided.

Many urinary tract infections (UTIs) go away on their own. According to some research, around 42% of UTIs clear up on their own during the first nine days. If your UTI symptoms persist or get worse, you should seek medical treatment.

Penicillin-type drugs, such as penicillin and amoxicillin, are some of the most commonly prescribed antibiotics. Other common classes of antibiotics include cephalosporins (such as cephalexin), macrolides (such as azithromycin), and lincosamides (such as clindamycin). All of them are used to treat bacterial infections, either by killing bacteria or preventing them from reproducing.

Antibiotics can be prescribed by a physician or other qualified healthcare provider at almost any walk-in clinic. Depending on the clinic and the type of antibiotic you need, you may be able to pick up your prescription on-site or at a local pharmacy. If you have a severe infection that requires intravenous (IV) antibiotics, you should seek emergency medical care.

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American Journal of Managed Care. What type of medication can an urgent care doctor prescribe?

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