Common Side Effects from Antibiotics, and Allergies and Reactions

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on Dec 11, 2024.

What are the most common side effects of antibiotics?

All medications have side effects, including antibiotics, but what are you most likely to see? Side effects of antibiotics can range from mild allergic reactions to severe and debilitating adverse events. When used appropriately, most antibiotics are relatively safe with few side effects. However, some side effects may interfere with your ability to finish the medication. In these cases, you should contact your doctor.

Antibiotics are medications that treat infections by killing bacteria or other organisms or slowing their growth. An antibiotic side effect occurs as an unwanted reaction that occurs in addition to the desirable therapeutic action of the antibiotic you are taking.

Common side effects with antibiotics include:

• Mild skin rash or other allergic reactions
• Soft or watery stools, short-term diarrhea
• Upset stomach, nausea
• Loss of appetite
• Fungal (yeast) vaginal infections or oral thrush

More severe antibiotic side effects include:

• Severe allergic reaction that results in difficulty breathing, facial swelling (lips, tongue, throat, face)
• Severe watery or bloody diarrhea; Clostridium difficile infection
• Stomach cramps
• Yeast infections in the mouth or vagina (white discharge and severe itching in the vagina or mouth sores or white patches in your mouth or on your tongue)

These side effects and how often they occur can vary. There are some common side effects that may occur within larger antibiotic drug classes, as described in Table 1. Long term side effects of antibiotics can occur, but are infrequent.

Should I stop my antibiotic if I'm having a side effect?

If you are experiencing a bothersome or serious antibiotic side effect, you should contact your health care provider right away to discuss your symptoms. The outcomes may include:

• Staying on the same antibiotic and managing the side effect
• Adjusting your dose
• Switching to a different antibiotic

In most cases, all antibiotic treatment should be finished unless your healthcare provider tells you otherwise. Stopping antibiotics early may allow the infection to worsen and may lead to antibiotic resistance, making the antibiotic less effective.

Even if the infection appears to have cleared up before all of the medication is gone, finish your treatment unless your doctor tells you to stop early.

Antibiotic allergies

Antibiotic allergies or hypersensitivity reactions are some of the most common side effects of antibiotics leading to emergency room admission.¹ Always tell your doctor of any previous allergic reaction to any medication, including antibiotics.

Mild allergic reactions may only result in a skin rash or itch. A more severe allergic reaction, called anaphylaxis, is a life-threatening medical emergency that requires immediate medical attention.

Signs of anaphylaxis may include:

• Shortness of breath
• Wheezing
• Severe nausea/vomiting
• Lightheadedness, dizziness
• Fast heart rate
• Swelling of the face, lips or tongue
• Shock

Call 911 right away for medical help if any of these symptoms should occur after taking an antibiotic.

Are antibiotics effective for a cold, flu or COVID-19?

Antibiotics are used to kill bacterial infections but they are not effective against viral infections, such as a cold, the flu, or COVID-19. They also do not work against fungal infections, like ringworm, thrush or vaginal yeast infections.

Your healthcare provider won't prescribe an antibiotic when you have a viral infection as it will not cure your infection and might make it worse. In addition, this adds to the problem of antibiotic resistance, may result in side effects, and can cost you money you do not need to spend. Your doctor can offer symptomatic treatment to ease your viral infection, or prescribe specific anti-viral medications if appropriate.

If you eventually need an antibiotic because of a secondary bacterial infection that might occur later, which can happen during a viral infection, your doctor will prescribe it then.

List of Antibiotics and Their Side Effects

There are several side effects that are common to most antibiotics, regardless of the group, class or drug. These side effects may include:

• antibiotic-associated diarrhea
• nausea, upset stomach
• yeast infections (vaginal, oral)
• anaphylaxis (a life-threatening allergic reaction)
• serious allergic skin reactions and other allergic reactions
• vein complications from intravenous (IV) use of antibiotics (phlebitis)

Table of Antibiotic Side Effects

The most common antibiotic drug classes and medicines are listed in Table 1, along with the most commonly reported antibiotic side effects (note that lists are not comprehensive). 

Table 1: Common Antibiotic Side Effects*

*This is not a complete list of common antibiotics or side effects that may occur. For a complete list of side effects, please refer to the individual drug monographs.

Related Questions

• Why Don’t Antibiotics Kill Viruses?
• Should you take probiotics with antibiotics?
• Antibiotic Resistance: The Top 10 List

Select antibiotic side effect descriptions

Allergic reactions, anaphylaxis: Allergic reactions account for the most common type of side effect with antibiotics.

• Previous research showed that 142,000 emergency department visits per year were due to antibiotic adverse events, and approximately 80% of these events were due to allergic reactions.
• Allergic reactions can typically only be prevented by avoiding the drug, although desensitization (for example, for penicillin) may be possible in certain circumstances for patients who have no other antibiotic options.¹
• Skin testing may be recommended for some instances of a reported penicillin allergy, when other drug classes are not optimal.²
• Anaphylaxis is the most serious type of allergic reaction and can be life-threatening.

Antibiotic-associated diarrhea: Antibiotic-associated diarrhea occurs in patients receiving antibiotics.

• About 5% to 25% of patients may develop antibiotic-associated diarrhea at any one time. The diarrhea occurs due to eradication of the normal gut flora by the antibiotic and results in an overgrowth of infectious bacteria, such as Clostridium difficile.
• If the diarrhea is severe, bloody, contains pus, or is accompanied by stomach cramps, fever or vomiting, a physician should be contacted.
• The most common antibiotics implicated in antibiotic-associated diarrhea are amoxicillin-clavulanate, ampicillin, and cefixime; however, other antibiotics may lead to this side effect, including cephalosporins, fluoroquinolones (e.g., ciprofloxacin, levofloxacin), azithromycin (Zithromax), clarithromycin (Biaxin XL), erythromycin, and tetracycline.³
• Probiotics such as Saccharomyces boulardii (Florastor) have been shown to be effective in helping to prevent antibiotic-associated diarrhea in children and adults.⁵

Vaginal yeast infections or oral thrush (candida species): Antibiotics may also change the normal flora balance in the vagina, often leading to an overgrowth of fungal species.

• Candida albicans is a common fungus normally present in small amounts in the vagina, mouth, gastrointestinal tract and on the skin and does not normally cause disease or symptoms. However, the fungus may take over when there is limited competition from bacteria due to antibiotic treatment.
• Thrush may appear as white patches in the mouth or on the tongue, and vaginal yeast infections produce a white discharge and intense itching.

Stevens Johnson Syndrome (SJS), Toxic Epidermal Necrolysis (TEN): Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN) are rare but serious allergic reactions to substances, often medications, that result in severe skin and mucous membrane disorders.

• Antibiotics such as sulfonamides, penicillins, cephalosporins, and fluoroquinolones may result in SJS and TEN.
• SJS and TEN can both cause rash, skin peeling, and sores on the mucous membranes and may be life-threatening.⁴
• TEN can lead to more than 30% loss of the skin surface and severe damage to the mucous membranes.

Injection site reactions or phlebitis: A reaction to an antibiotic can occur if the antibiotic is given intravenously (IV) into a vein.

• Injections site reactions and phlebitis (vein inflammation) can occur. The vein and area with the IV needle may be red, swollen and hot. An infection may or may not be present.
• Typically, the needle must be removed and reinserted elsewhere to help clear the injection site reaction.

Antibiotics are among the most commonly prescribed medications in the US. However, many side effects may not be reported. Always consult your doctor or healthcare specialist for medical advice.

You may also report side effects to the FDA online or call 1-888-INFO-FDA (1-888-463-6332). If you have a medical emergency call 911.

This is not all the information you need to know about antibiotic side effects and does not take the place of your doctor’s directions. Review the full product information and discuss this information and any questions you have with your doctor or other health care provider.

See also

• Antibiotic Resistance: The Top 10 List
• Antibiotics and Birth Control Pill Interactions
• Antibiotics for UTI Treatment
• Can You Drink Alcohol with Antibiotics?
• Middle Ear Infection FAQs (Acute Otitis Media)
• Why Don’t Antibiotics Kill Viruses?

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Medical Disclaimer