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Common Side Effects of Antibiotics

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Common Side Effects of Antibiotics

writtenByWritten by: Dr. Heidi Lightfoot
Dr. Heidi Lightfoot

Dr. Heidi Lightfoot

Medically reviewed by Dr. Heidi Lightfoot, MD who is an anesthesiologist in Hampshire, United Kingdom. Alongside her clinical work, she has an interest in medical writing and clinical research.

Read more posts by this author.

December 10, 2020 Read Time - 10 minutes

Common Side Effects of Antibiotics

Antibiotics have become one of the most important tools of modern medicine. They offer an easy and effective means of treating infections that could otherwise be a serious threat to your health and make you very unwell. While they can make you feel better when you are sick, antibiotics can sometimes come with side effects. Let’s take a look at some common side effects of antibiotics and how you can reduce or prevent them.

What are Antibiotics?

Antibiotics, sometimes called antibacterials, refer to any medication that can effectively kill bacteria or stop the bacteria from reproducing, thus treating infections caused by bacteria.

Antibiotics were originally discovered in 1928, when Scottish doctor Alexander Fleming observed that a certain type of mold known as Penicillium notatum stopped the growth and spread of a type of bacteria called Staphylococcus. Staphylococcus bacteria can cause pneumonia, skin infections, and certain food-borne illnesses.

Further studies found that the mold could also kill Streptococcus, Diphtheria bacillus, Meningococcus, and other types of bacteria. From this research the first ever antibiotic, penicillin, was developed.

How Do Antibiotics Work?

The antibiotics you are prescribed will depend on the type of bacteria causing the infection. Bacteria are generally divided into two groups, depending on the make-up of their cell walls:
Gram-positive bacteria have single-layered cell walls that are thin and easy to penetrate.
Gram-negative bacteria have thicker, double-layered cell walls that are not easily permeable.

To tackle these two types of bacteria, antibiotics are split up into several groups. In general antibiotics can be thought of as ‘broad spectrum’ or ‘narrow spectrum’, depending on how many bacteria and types of infection they are effective against.

  • Broad spectrum antibiotics are designed to work against a wide variety of bacteria, both gram-positive and gram-negative. Broad spectrum antibiotics include quinolones and tetracyclines.
  • Narrow spectrum antibiotics are only effective against either gram-positive or gram-negative bacteria but not both. Narrow spectrum antibiotics include penicillin and clindamycin.

Antibiotics use a variety of different mechanisms to eliminate or neutralize bacteria, but they generally work by:

  • Directly killing bacteria by weakening their cell walls until they burst and die
  • Inhibiting vital process that allow bacteria to grow and spread new cells
  • Hampering a bacteria’s ability to repair any damage to its DNA, such that it dies

For example, quinolones, which are a type of broad-spectrum antibiotic, use hydroxyl radicals to eliminate bacteria. Hydroxyl radicals are molecules that destroy the proteins and lipids that make up the cell membrane in bacteria. These molecules can also damage cell DNA to prevent the bacteria cells from replicating.

Other antibiotics, like penicillin, eliminate bacteria by destroying the cell wall. Without a cell wall, bacteria can’t keep themselves together, forcing them to burst and die.

Why Antibiotics Cause Side Effects

Your body is naturally filled with millions of bacteria. Most of them are harmless, and many are actually beneficial or necessary for your health and general processes. Your gut bacteria, for instance, help you digest food properly.

Antibiotics can kill bacteria effectively, but they can’t always differentiate between good bacteria and bad bacteria. This means they may damage bacteria that you need, which can keep your body from operating properly while allowing the foreign bacteria greater reign over your organ systems.

Common Side Effects of Antibiotics

Some of the most common side effects you may experience with antibiotics include:

  • Upset stomach – Many antibiotics can cause an upset stomach or other general gastrointestinal problems. This is most common with penicillins, cephalosporins, macrolides, and fluoroquinolone antibiotics. Symptoms include:
    • Nausea
    • Vomiting
    • Diarrhea
    • Stomach cramps
  • Photosensitivity – Certain antibiotics, like tetracycline, can make you more sensitive to light. This can make your skin more susceptible to sunburn, and your eyes may be more sensitive to natural light.
  • Fever – Fever is a common side-effect of many medications, not just antibiotics. . Fever can also happen as part of an allergic reaction (more on this later). Fevers are more common with sulfonamides, cephalexin, beta lactams, and minocycline, but they can happen with any antibiotic.
  • Tooth discoloration – Children whose teeth are still developing may suffer from permanent tooth staining when using antibiotics such as doxycycline and tetracycline. This is generally more common in children younger than eight years old.
    Women who take antibiotics while pregnant may also stain the teeth of their developing child, so pregnant women should not take these types of antibiotics
  • Fungal infections – Because antibiotics kill off lots of the healthy, protective bacteria in your body, they can cause certain fungal infections, particularly thrush. Thrush often appears as white, painful patches in your mouth and on your tongue. Antibiotics can also cause vaginal yeast infections, which can cause itchiness, pain, and general discomfort along with a fluid discharge. You might need treatment for these fungal infections and should seek advice from a doctor if you develop these symptoms.
  • Interactions with Other Medications – Some antibiotics may affect the effectiveness of other medications you are taking. For example, the antibiotic rifampin can affect the effectiveness of the birth control pill.

Read: Do Antibiotics Make You Tired?

Serious Antibiotics Side Effects

Most side effects of antibiotics are not serious and should subside once you finish the course. However, depending on the sensitivity of the side-effects and your personal health, you may experience some more serious side effects, though these are rare.

Allergic reactions can happen with antibiotics and any medication. Allergic reactions to antibiotics can be relatively mild, but some can be very serious and require immediate medical attention.

If you are allergic to a certain antibiotic, you will react to it almost immediately after administration. Symptoms differ based on the person, but most allergic reactions manifest in the form of hives, trouble breathing, and swelling in the tongue and throat. If you have a severe allergic reaction to an antibiotic, known as ‘anaphylaxis’, this can be life-threatening. If you develop these symptoms you should seek medical advice immediately.

Another type of allergic reaction that can occur with antibiotics is Stevens-Johnson syndrome. Stevens-Johnson syndrome is a serious illness affecting your mucus membranes and skin. It is most common with sulfamethoxazole and beta-lactam antibiotics. Stevens-Johnson syndrome starts out with flu-like symptoms, including sore throat and fever and can progress to a painful rash that quickly spreads and causes blisters. This may be followed by the top layer of your skin shedding.

Other symptoms of Stevens-Johnson syndrome include:

  • Skin pain
  • Hives
  • Coughing
  • Swelling in your tongue or face
  • Pain in your throat and mouth

If you develop signs and symptoms of Stevens-Johnson syndrome you should stop taking the antibiotic immediately and seek medical advice straight away.

In rare instances, antibiotics can cause changes to your blood. For instance, some antibiotics can reduce the number of white cells in your blood (leukopenia). This can cause a weakened immune system and a higher chance of infections. Some antibiotics may cause low levels of platelets in your blood (thrombocytopenia). Platelets are needed for you body to form clots if you cut yourself, so not having enough can slow down your blood clotting and cause problems with bruising and bleeding.

Some antibiotic medications, usually fluoroquinolone antibiotics such as ciprofloxacin, may also irritate or inflame your tendons. Tendons are the thick tissues that connect your muscles to your bones. Continued irritation can potentially lead to tendonitis or a ruptured tendon. This is more common in people who:

  • Suffer from existing kidney failure
  • Have undergone a lung, heart, or kidney transplant
  • Experienced past problems in their tendons
  • Are 60 years old or older
  • Take steroids

In the rarest cases, antibiotics can cause low blood pressure, irregular heartbeat, and other cardiovascular problems. These side-effects have been linked to erythromycin and some types of fluoroquinolone antibiotics.

To read about penicillin allergy click here
To read about amoxicillin allergy click here

Reducing and Preventing the Side Effects of Antibiotics

The best thing you can do to reduce the potential for antibiotics side effects is to take them as the label and follow your doctor’s instructions. Most antibiotics should be taken with food, as this can help to improve absorption and prevent an upset stomach. However, some must be taken with just water on an otherwise empty stomach – always read the information provided with the antibiotic you are prescribed. Talk to your doctor about ways to prevent upset stomach if you cannot take antibiotics with food.

Most gastrointestinal problems (diarrhea and soft stools) come from the fact that the antibiotics are killing the good bacteria in your gut, which disrupts the balance. There is some evidence to suggest that taking probiotics can help to replenish your gut flora and regain the bacteria balance in your digestive tract.

While the role of probiotics needs more research, you will certainly do no harm by taking a probiotic supplement or eating foods rich in probiotics, such as yogurt, sauerkraut, kimchi, kombucha, and most fermented foods.

While taking antibiotics you should avoid alcohol. Although alcohol is a fermented beverage, is does not have the same beneficial properties of other probiotic foods. Alcohol can in fact cause some of the same side effects as antibiotics, and so putting them together can make you feel much more unwell. Certain antibiotics, such as metronizadole, trimethoprim, and sulfamethoxazole, can cause severe reactions when combined with alcohol, leading to symptoms like:

  • Flushing or redness in the face
  • Increased heart rate
  • Nausea and vomiting
  • Headaches

Completing the Course of Antibiotics

Make sure you take the complete course of antibiotics as directed by your doctor to completely eliminate the bacteria from your system and treat the infection properly. Even if it seems like your symptoms have cleared up, there may still be lingering bacteria.

Stopping antibiotics early gives these bacteria a chance to grow and spread again, this time with the higher chance of becoming resistant to antibiotics. This could lead to more severe symptoms and require more intense antibiotics that could cause more serious side effects.

You should also only take the dose prescribed by your doctor, and do not increase your dosage unless your doctor instructs you to do so. Most antibiotics will take effect within the first few hours of administration, and you should feel better within the first day. If you do not feel better or if your symptoms do not improve within the first few days, contact your doctor. You may require a different antibiotic or may have been misdiagnosed.

How to Obtain Antibiotics

Getting the antibiotics you need is a relatively easy process, but you do need a prescription for them. Start by contacting your doctor. They will perform a physical examination and ask about your symptoms and personal health history. From there, your doctor may have enough information to provide a diagnosis and prescribe any necessary medication.

However, your doctor may need to order lab tests to get a better idea of the type of bacteria that is causing your illness. They may take a sample, often by swabbing your cheeks or drawing blood, and send it to a lab, where tests are run to identify the specific bacteria. Once the doctor has the results, they can prescribe the antibiotic that will target that bacteria and treat your infection most effectively

Despite the potential for side effects, if you have an infection and need antibiotics, the benefits far outweigh the risks. If you have symptoms of an infection and think you need antibiotics, you can get antibiotics online from a professionally trained and certified doctor by scheduling an appointment with PlushCare today.

Read More About Common Side Effects of Antibiotics

Read More of Our Antibiotics Series


PlushCare is dedicated to providing you with accurate and trustworthy health information.

NIH. Side Effects of Antibiotics. Accessed on January 29, 2021 at

Cleveland Clinic. Antibiotics. Accessed on January 29, 2021 at

CDC. Do antibiotics have side effects? Accessed on January 29, 2021 at

Most PlushCare articles are reviewed by M.D.s, Ph.Ds, N.P.s, nutritionists and other healthcare professionals. Click here to learn more and meet some of the professionals behind our blog. The PlushCare blog, or any linked materials are not intended and should not be construed as medical advice, nor is the information a substitute for professional medical expertise or treatment. For more information click here.

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