How Fast Do Antibiotics Work? (2023)

Antibiotics are medications used to treat infections that are caused by bacteria. There are several classes of antibiotics—some that only target specific bacteria and others that can be used against a wide range of organisms.

This article will explore how antibiotics work, what they are used to treat, and how quickly they can help end an infection.

Choosing the Right Antibiotic for Bacterial Infections

How Fast Do Antibiotics Work? (1)

What Is An Antibiotic?

Antibiotics are substances that get their start in nature, usually as fungi or other forms of bacteria that exist in soils. These substances are able to bind to the cell walls of harmful bacteria, penetrating the cell to either kill the bacteria or prevent it from reproducing.

Penicillin was the first antibiotic developed, and it happened by accident.

British scientist Alexander Fleming developed penicillin in the late 1920s after noticing that a mold growing on the same plate as Staphylococcus aureas bacteria was destroying the bacteria. He determined that the Penicillum mold created a substance that could dissolve bacteria, and over the next several decades penicillins were increasingly used to treat a variety of infections.

Antibiotic development exploded after World War II, and today there are several classes of antibiotics to choose from. Each has its own place in treating bacterial infections, with some used to attack a wide range of bacteria while others have more specialized uses.

Choosing the Right Antibiotic for Bacterial Infections

How Do Antibiotics Work?

Antibiotics work by binding to bacterial cells and penetrating their cells walls. Once inside the bacterial cell, these medications either kill the bacteria or prevent it from being able to reproduce and grow. Antibiotics are divided into classes based on how they enter cell walls and destroy bacteria. This is called the mechanism of action.

Antibiotics by Mechanism of Action

Different types of antibiotics work against bacteria in different ways. Below are some examples.

Antibiotics that destroy cell walls

(Video) How antibiotics work

  • Beta-lactam antibiotics
  • Glycopeptides

Antibiotics that alter genetics of the bacteria

  • Tetracyclines
  • Macrolides
  • Oxazolidinones

Antibiotics that prevent bacteria from reproducing

  • Quinilones
  • Sulfonamides

Some antibiotics work best on certain types of bacteria. Mainly, these are classified as antibiotics that target gram-positive bacteria with a simple cell wall, and those that target the more complex gram-negative bacteria.

Your healthcare provider may treat you with a broad-spectrum antibiotic that works against both gram-positive and gram-negative bacteria, but some infections may require a culture test to identify the specific form of bacteria. By identifying the exact type of bacteria that's causing your illness, your healthcare provider can prescribe you the form of antibiotic that's most effect against that particular bacteria.

How Long Do They Take to Work?

How long antibiotics take to work depends on the infection being treated and what type of antibiotic you are taking. Some antibiotics achieve their maximum effect with high concentrations, and these may be given in the form of intravenous infusions. Other types of antibiotics are more effective over time.

For example, levofloxacin is in the fluoroquinolone class of antibiotics and it reaches its peak performance based on concentration. It is a stronger antibiotic that begins to work in hours, but can take days to completely cure an infection.

On the other hand, penicillins work over a period of time. Depending on how your illness has spread, it can take days or even weeks to work completely. For example, when used to treat strep throat, penicillins can make you non-infectious to others in about a day, but you may need to keep taking medication for several days to resolve your own infection.

Always take antibiotics for the full regimen prescribed by your doctor. Stopping a course of antibiotics early can worsen your infection or increase your resistance to antibiotics overall. Even if you start to feel better, some antibiotics continue to work for awhile after your stop taking them, and how quickly they work can vary from person to person.

How Long Do Antibiotics Stay in Your System?


Antibiotics only work against bacterial infections, and certain antibiotics are only effective at fighting very specific types of bacteria.

Gram-positive bacteria have a simple cell wall, but gram-negative bacteria are surrounded with an additional layer that is more difficult for antibiotics to penetrate. Antibiotics are usually prescribed as either broad-spectrum—meaning they can treat both types of bacteria, or they are prescribed specifically for the type of bacteria that is causing your infection.

Common antibiotics are listed below based on what type of bacteria they work against, and some of the common infections they might be used to treat. For example, penicillins and vancomycins can only be used to treat infections caused by gram-positive bacteria. This includes things like throat infections, C.diffinfections, and neurosyphilis—an infection of the brain and spinal cord.

Other antibiotics like amoxicillin, ampicillin, cephalosporins, and fluoroquinolones can treat a variety of both gram-positive and gram-negative infections including sepsis and urinary tract infections.

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Antibiotics Are Not a Cure-All

Antibiotics should only be used for specific bacterial infections. Taking antibiotics only when appropriate can help fight antimicrobial resistance. Antibiotics DO NOT work against viruses and cannot be used to treat infections like influenza or COVID-19.

You should also be sure to always take your full course of antibiotics. Don't save them in case you get sick later—antibiotics you take for one infection may not work on another. You should also never share your antibiotics with anyone else or use antibiotics that were not prescribed to you.

(Video) Bacteria filmed 'hiding' from antibiotic

Side Effects

Side effects of antibiotics can vary depending on the type of antibiotic you are taking, what dose you were prescribed, and what other health issues you may have. Some other medications can affect how well your antibiotics work and what side effects you develop.

Alcohol and Antibiotics

Alcohol use is not recommended to be taken with most medications—including antibiotics—over concerns that alcohol use with these medications can reduce how well they work and increase their toxicity. There are warnings against alcohol use with several specific types of antibiotics like doxycycline and cephalosporins, but the science behind these recommendations has recently come into question.

A 2020 paper reviewed the data behind these recommendations and found that most studies of combined alcohol and antibiotic use were limited and focused mainly on antibiotic use in alcoholics and people who consumed large amounts of alcohol. Few studies investigated the effect of social or limited alcohol consumption while taking antibiotics.

While the paper raised questions about the true dangers of alcohol use with antibiotics, the authors recommended that alcohol should still be avoided with antibiotic use until more research can be done.

Most side effects of antibiotics stem from the fact that they may not only target the bacteria causing your infection. Antibiotics can kill "good" bacteria and upset the natural balance in your body, especially in the gut. The use of probiotics while taking antibiotics may helpful in reducing some of these side effects.

Common side effects of antibiotics include:

  • Rashes
  • Nausea
  • Diarrhea
  • Yeast infections

If you experience things like shortness of breath, hives, or other symptoms of allergic reaction, stop taking your antibiotic and call you healthcare provider immediately.

Common and Serious Antibiotic Side Effects in Children

Antibiotic Resistance

Antibiotic resistance is a problem that emerges because of overuse and misuse of antibiotics. When this happens, it's not your body that becomes resistant to antibiotics. Instead, the bacteria antibiotics are meant to kill or damage are no longer effected by the medications.

You can help avoid antibiotic resistance by taking antibiotics only as directed by your doctor, and only when you really need them. Antibiotics don't work for everything and are not effective against viruses. The Centers for Disease Control and Prevention estimates that about one-third of the antibiotics prescribed each year in the United States are unnecessary and are given for conditions that should not be treated with antibiotics.

U.S. Updates Antibiotic Resistance Plan: Here's What's New

(Video) What does antibiotic resistance look like? Watch this experiment.

When to See a Healthcare Professional

You should only take antibiotics under the direction of a healthcare professional. If your symptoms worsen or aren't resolved after your full course of antibiotics, talk to your healthcare provider about additional treatment options.

You should also alert your healthcare provider to any side effects you experience while taking antibiotics, even if they are minor. Go the emergency room or seek immediate medical care if you experience symptoms of an allergic reaction while taking antibiotics.

Symptoms of Anaphylaxis

A Word From Verywell

Antibiotics are an extremely useful tool in medicine and help treat all kinds of infections that once would have been fatal. Antibiotics work quickly, but how quickly will depend on the type you are taking and what condition you are being treated for. Always take your antibiotics for the full time period prescribed by your doctor, and exactly as the medication is prescribed. Stopping antibiotics early, or taking them inappropriately can lead to antibiotic resistance and make you more susceptible to dangerous infections.

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Frequently Asked Questions

  • Can you drink on antibiotics?

    It's not recommended to drink alcohol with antibiotics—and particularly with certain varieties. Talk to your doctor of pharmacist about the specific antibiotic you were prescribed and how it may interact with alcohol. It's also a good idea to review the effects of alcohol on any other medications you may be taking, too.

    Learn More:Zorvolex (Diclofenac) and Alcohol

  • What happens when you drink on antibiotics?

    While the science behind the recommendation to avoid drinking alcohol while taking antibiotics has been debated, its generally believed that alcohol can reduce how well antibiotics work and increase toxicity levels. You may experience things like increased nausea or vomiting when combining antibiotics with alcohol.

    (Video) How do antibiotics work and where do they come from?
  • How long does it take for a UTI to go away without antibiotics?

    A urinary tract infection may resolve on its own without the use of antibiotics, but antibiotics can help decrease the length of your infection and help prevent complications. A UTI can be treated within a few days with antibiotics, but can last much longer without. Even worse, an untreated UTI can lead to more severe illness like kidney infections or even urosepsis.

    Learn More:Symptoms of a Urinary Tract Infection

  • How long do antibiotics stay in your system?

    How long antibiotics stay in your system depends on the type of antibiotic you are taking. Some last as little as a few hours after your last dose while others can stay in your system for weeks. Types of antibiotics that last the longest in your body include certain types of penicillins and hydroxychloroquine.

    Learn More:Antibiotics for Treating Infections


1. How are antibiotics chosen for chest infections?
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2. Exploding bacteria with penicillin
3. How to take antibiotic correctly
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4. When Not to use Antibiotics
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5. The Science of Antibiotics: How They Work and Why They Matter
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6. Myth or Truth? Stop taking Antibiotics when you feel better.
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