Types of Antibiotics: A Guide

Medically Reviewed By Alan Carter, Pharm.D.
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More than 100 antibiotics are in use today. Different types of antibiotics treat different bacterial infections. The discovery of the first true antibiotic — penicillin — in 1928 was one of the most life changing events of the 20th century. Before its discovery, doctors could not do much to help people with bacterial infections. People died from illnesses and injuries that are highly curable today. Now, more than 100 antibiotics are available to treat different bacterial infections.

Here is a look at common antibiotic names and the types of antibiotics your doctor may prescribe.


A closeup of a person's arm, with the hand holding a bottle of pills. The arm is covered in sticky notes with the names of the different antibiotic classes on them.

The first penicillin gave rise to an entire class of antibiotics known as penicillins. They are derived from a specific mold or type of fungi called Penicillium.

These widely useful antibiotics are often a doctor’s first choice for several types of infections, including:

They are highly effective against familiar organisms such as staph and strep.

Common side effects of penicillins include:

In rare cases, people with an allergy to penicillin may experience anaphylaxis, a severe allergic reaction. This is considered a medical emergency and requires immediate care.

Anaphylaxis is a severe reaction to a trigger, such as an allergen, and is potentially life threatening. 

Symptoms can come on quickly and include: 

If you or someone around you develops these symptoms, it’s important to: 

  1. Check to see whether they have an epinephrine pen. If they do, read and follow the instructions to dispense the medication.
  2. Dial 911 or a local emergency number.
  3. Lay them down. If they have vomited, lay them on their side. 
  4. Stay with them until emergency services arrive. 

Someone may need more than one injection with an epinephrine pen. If symptoms do not begin to clear after 5–15 minutes, give a second injection if one is available.

Examples of penicillins include:

  • amoxicillin
  • ampicillin
  • penicillin G
  • penicillin V
  • flucloxacillin

Read about penicillin allergies.


Cephalosporins are related to penicillins. Both belong to a larger class called beta-lactams. Like penicillins, cephalosporins originally came from a fungus — Cephalosporium.

There are five generations of cephalosporins. Each generation covers different types of bacteria. As a result, the class can treat a variety of infections, from strep throat and skin infections to very serious infections such as meningitis. Because cephalosporins are related to penicillins, some people with a penicillin allergy may also react to cephalosporins. 

Other common side effects include:

Examples of cephalosporins include:

  • cefixime
  • cefpodoxime
  • cefuroxime
  • cephalexin


Macrolides are a different class of antibiotics from beta-lactams. However, they effectively treat many of the same infections, such as:

  • respiratory infections
  • ear infections
  • skin infections
  • sexually transmitted infections

Macrolides are a useful alternative for people with allergies to beta-lactams. They are also useful when bacteria develop resistance to beta-lactam antibiotics.

However, macrolides have a lot of possible interactions with other medications. Be sure your doctor and pharmacist know about any other medications you are taking when they prescribe a macrolide. 

Common side effects include:

Examples of macrolides include:

  • azithromycin
  • clarithromycin
  • erythromycin


Fluoroquinolones, or quinolones, are active against a very wide variety of bacteria. They are a family of broad-spectrum antibiotics. This makes them useful for treating infections when other antibiotics are not effective.

They are also an alternative for people who have allergies to other types of antibiotics. They can treat infections such as:

However, this class of antibiotics can cause issues for people who are predisposed to heart arrhythmias. Be sure your doctor and pharmacist know your complete medical history before they prescribe this type of medication. 

Common side effects include:

Examples of fluoroquinolones include:


Sulfonamides are bacteriostatic, which means they stop bacterial growth and your immune system does the rest. In this way, they differ from other types of antibiotics, which kill the bacteria on their own.

Sulfonamides can be used to treat a variety of conditions, including:

Common side effects include:

  • diarrhea
  • nausea
  • vomiting
  • yeast infection
  • headaches 

Large doses of sulfonamides can cause severe allergic reactions.

Examples of sulfonamides include:

  • sulfacetamide
  • sulfadiazine
  • sulfamethoxazole-trimethoprim


Tetracyclines are broad-spectrum antibiotics. Like sulfonamides, they are bacteriostatic.

They can be used to treat various infections and conditions, including:

Common side effects include:

  • stomach pain or upset
  • pain in the upper abdomen, below the rib cage
  • vomiting
  • sensitivity to light

Examples of tetracyclines include:

Other types of antibiotics

Doctors have several other antibiotic choices if none of these classes will work. They will use some of them only in a hospital. Some antibiotics do not fit into the main groups, including clindamycin, metronidazole (Flagyl), and nitrofurantoin (Furadantin, Macrodantin). 

Each antibiotic, whether in a defined class or not, has different dosing requirements. You need to take some of them on an empty stomach and others with food. Ask your doctor or pharmacist about the best way to take an antibiotic.

With all antibiotics, it’s important to finish the entire course your doctor prescribes. This ensures adequate treatment and prevents antibiotic resistance.

What are antibiotics, and how do they work?

Antibiotics are medications that help your body fight bacterial infections. They are effective in both people and animals.

Different types of antibiotics work in different ways. Typically, they work in one of the following ways:

  • preventing cell reproduction
  • changing the necessary function within the cell
  • changing the process within the cell

You can take antibiotics in different ways as well, including:

  • orally
  • topically
  • via injection
  • intravenously, or directly into a vein

What are antibiotics typically prescribed for?

Doctors can prescribe antibiotics for various bacterial infections. They commonly use antibiotics to treat infections such as:

Sometimes doctors may use antibiotics to treat ear infections and sinus infections. However, this is not always necessary. It is important to take antibiotics only when you actually need them. Taking them when they are not necessary will not help. Also, most antibiotics can cause side effects.

Always take antibiotics exactly the way your doctor instructs you to. It is also important to complete the entire cycle of antibiotics, even if you begin to feel better.

Frequently asked questions

These are some questions people frequently ask about antibiotics. They have been reviewed by Alan Carter, PharmD.

What’s the strongest antibiotic? 

While penicillin is one of the most commonly used antibiotics globally, it may not be the strongest. An antibiotic’s effectiveness typically depends on the type of infection it is treating. Certain types of antibiotics are more effective on specific infections than others.

What are the classes of antibiotics?

The classes of antibiotics are:

  • penicillins
  • macrolides
  • cephalosporins
  • fluoroquinolones
  • tetracyclines
  • sulfonamides
  • urinary anti-infectives
  • lincosamides


Many different types of antibiotics are available. Doctors prescribe them mainly to treat bacterial infections.

Not all antibiotics are effective on the same infections. Taking antibiotics when you do not need them is not helpful to you, and you may experience side effects.

Always take antibiotics exactly as your doctor instructs you to. Complete the entire cycle of antibiotics, even if you begin to feel better.

Tell your doctor about any side effects you experience.

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Medical Reviewer: Alan Carter, Pharm.D.
Last Review Date: 2022 Nov 30
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THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.
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