Skip to main content

Common Side Effects from Antibiotics, and Allergies and Reactions

Medically reviewed by Leigh Ann Anderson, PharmD. Last updated on Sep 15, 2021.

Common Side Effects | Should I Stop? | Allergies | Antibiotics for Cold, Flu or COVID? | Antibiotic List and Side Effects | More Resources

What Are the Most Common Side Effects of Antibiotics?

All medications have side effects, including antibiotics. 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.

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.

Common side effects with antibiotics include:

  • Mild skin rash or other allergic reactions
  • Soft 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 are extremely variable; however, 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 to discuss your symptoms. The outcomes may include:

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

In most cases, all antibiotic treatment should be finished unless your healthcare prover 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.

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.

Anaphylactic reactions due to antibiotics may include:

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

Immediately call 911 for medical help if any of these symptoms should appear after taking an antibiotic.

Are Antibiotics Effective for a Cold, Flu or COVID?

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

You should avoid demanding an antibiotic from your healthcare provider 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, and it costs 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, 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 class or drug. These side effects may include:

  • antibiotic-associated diarrhea
  • yeast infections (vaginal, oral)
  • anaphylaxis
  • serious allergic skin reactions, other allergic reactions
  • complications from intravenous (IV) use of antibiotics (phlebitis)

What are the side effects of antibiotics? The most common antibiotic classes and drug members are listed in Table 1, along with the most commonly reported antibiotic side effects (list are not comprehensive).

Table 1: Common Antibiotic Side Effects*

Common Antibiotic Classes Antibiotic Class Members Most Common Class Side Effects Additional Clinical Comments
List of penicillins, penicillinase- resistant penicillins, and other penicillin-type drugs
  • skin rash
  • diarrhea
  • stomach pain
  • nausea and vomiting
  • drug fever
  • hypersensitivity (allergic) reactions

If bloody stools, an extreme watery diarrhea, stools with pus, anaphylaxis (a severe allergy), urgent stomach pain, severe skin reaction, or fever occur contact health care provider immediately.

Antibiotics may cause life-threatening pseudomembranous colitis and Clostridium difficile infection.

List of cephalosporins
  • rash
  • diarrhea
  • nausea and vomiting (rare)
  • hypersensitivity (allergic) reactions
  • serum sickness
  • vaginal candidiasis (yeast infection)

Cross-hypersensitivity may occur in patients with documented penicillin allergy; may be more common with first generation cephalosporins due to structural similarities.

In one prospective study2, the rate of cross-reactivity among subjects with a positive penicillin skin test was 6%; however rates up to 10% have been reported.

If you have a history of penicillin allergy, your doctor may recommend penicillin skin testing if a cephalosporin is required.

List of monobactams

The monobactams have a unique structure and are different from other beta-lactam antibiotics (eg, penicillins, cephalosporins, cephamycins).

Cayston (aztreonam) inhalation is used in people who have cystic fibrosis and a certain bacteria in their lungs. This helps to improve their breathing symptoms.

Aztreonam (Azactam, IV)

  • nausea, vomiting
  • diarrhea
  • rash
  • vaginal itching or discharge
  • pain, bruising, swelling, or irritation where the medicine was injected

Aztreonam (Cayston, inhalation)

  • cough, wheezing
  • nasal congestion,
  • throat pain
  • fever
  • chest discomfort
  • stomach pain and vomiting

While cross-reactivity of aztreonam (Azactam) with other beta-lactam antibiotics is rare, use with caution to any patient with a history of hypersensitivity to beta-lactams (eg, penicillins, cephalosporins, and/or carbapenems).6,8

Treatment with aztreonam can result in hypersensitivity reactions in patients with or without prior exposure.

List of aminoglycosides
  • renal (kidney) toxicity
  • ototoxicity (hearing loss)
  • dizziness
  • nausea and vomiting
  • nystagmus (involuntary eye movement)

Long-term aminoglycosides or multiple treatment periods may lead to greater risk for ototoxicity (hearing damage, loss) and renal (kidney) toxicity.

Aminoglycosides are often reserved for times when less toxic antibiotics cannot be used or are ineffective.

Aminoglycosides are not well absorbed by mouth, and are usually given by injection. Some products are given by inhalation for lung infection.

Neomycin is given by mouth for its effects in the intestine, although it can be absorbed and toxic reactions may occur.

List of carbapenems
  • diarrhea
  • nausea and vomiting
  • headache
  • rash
  • hepatic (liver) toxicity
  • eosinophilia (high levels of a type of white blood cell)
Hypersensitivity reactions reported with meropenem and imipenem in patients with penicillin allergy.
List of antituberculosis agents
  • diarrhea
  • nausea and vomiting
  • anorexia
  • hemolytic anemia
  • liver toxicity
  • headache
  • peripheral neuropathy
  • dizziness
  • reddish-orange body fluids (with rifampin, rifabutin only)

Sides effects vary among agents, check each individually.

Vitamin B6 (pyridoxine) may be taken to help prevent peripheral neuropathy with isoniazid.

List of glycopeptides
  • vancomycin: "red man syndrome" (RMS) - flushing, hypotension, itching with IV use; phlebitis
  • telavancin: taste alteration, nausea/vomiting, headache, dizziness

IV infusion of vancomycin over 60 minutes may help to prevent RMS.

Other cases of RMS due to other antibiotics have been reported, including: rifampin, cefepime, teicoplanin, ciprofloxacin, and amphotericin B.7

List of macrolide antibiotics
  • abdominal pain
  • diarrhea
  • appetite loss
  • nausea and vomiting
  • taste alterations (clarithromycin)

High rate of gastrointestinal (stomach) side effects.

Do not crush, chew, break, open enteric-coated or delayed-release pills.

List of sulfonamides (antibiotic)
  • nausea and vomiting
  • diarrhea
  • anorexia (appetite loss)
  • abdominal (stomach pain)
  • rash
  • headache
  • dizziness
  • photosensitivity

Avoid prolonged sunlight exposure; use sunscreen, and wear protective clothing.

Sulfonamide allergic reactions have been reported in roughly 1.5% to 3% of the general population. Learn more about sulfa allergies here.

May lead to severe skin reactions: Stevens Johnson Syndrome, Toxic Epidermal Necrolysis.

List of tetracyclines

  • nausea and vomiting
  • diarrhea
  • anorexia
  • abdominal (stomach) pain
  • tooth discoloration in children < 8 years
  • liver toxicity
  • photosenstivity

Avoid prolonged sunlight exposure, use sunscreen, wear protective clothing.

The development of bacterial resistance has limited the effectiveness of this class of drugs, although they may still be used in human and animal medicine.

List of fluoroquinolones (quinolones)
  • nausea and vomiting
  • diarrhea
  • abdominal (stomach) pain
  • headache
  • lethargy
  • insomnia (difficulty sleeping)
  • photosensitivity (can be severe)

Due to a risk for serious adverse reactions, doctors may withhold use of this class unless absolutely required for more serious or unresponsive infections.

Avoid prolonged sunlight exposure; use sunscreen, wear protective clothing.

See FDA alerts and boxed warnings for fluoroquinolones: tendon rupture, tendonitis, peripheral neuropathy, aggravation of myasthenia gravis, aortic aneurysm or dissection, low blood sugar, mental status changes.

List of lincomycin derivatives
  • pseudomembranous colitis (may be severe and life-threatening)
  • diarrhea
  • nausea and vomiting
  • rash
  • allergies
  • jaundice (clindamycin)
If severe diarrhea during treatment or for up to 8 weeks after treatment consult health care provider immediately, may be pseudomembranous colitis (C. difficile); consider use of less toxic agents.
Miscellaneous antibiotics
  • metallic taste
  • nausea and vomiting
  • dizziness
  • headache
  • vaginal yeast infections

Avoid alcohol use and or propylene glycol use during treatment and for up to 3 days after treatment stopped.

Combined use with alcohol may lead to cramps, nausea/vomiting, flushing, headache; may discolor urine red-brown.

*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.

Side Effects

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 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., side effects of Cipro antibiotic), azithromycin (e.g., Azithromycin, Z Pak), clarithromycin (Biaxin), 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

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 or call 1-800-FDA-1088 (1-800-332-1088).

See also


  1. Shehab N, Patel P, Srinivasan A, et al. Emergency department visits for antibiotic-associated adverse events. Clinical Infectious Diseases 2008;47:735-43.
  2. Park MA, Koch CA, Klemawesch P, Joshi A, Li JT. Increased adverse drug reactions to cephalosporins in penicillin allergy patients with positive penicillin skin test. Int Arch Allergy Immunol. 2010;153(3):268-273. doi: 10.1159/000314367.
  3. Bartlett JG. Clinical practice. Antibiotic-associated diarrhea. N Engl J Med 2002:346:334-9.
  4. The Merck Manual for Health Care Professionals. Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN).
  5. Szajewska H, KoƂodziej M. Systematic review with meta-analysis: Saccharomyces boulardii in the prevention of antibiotic-associated diarrhoea. Aliment Pharmacol Ther. 2015 Oct;42(7):793-801. doi: 10.1111/apt.13344. 
  6. Eljaaly K, Stevens R. Penicillin Allergies and Cross-Reactivity With Other Beta-Lactams. May 17, 2017. Pharmacy Times. Health System Edition.
  7. Martel TJ, Jamil RT, King KC. Red Man Syndrome. [Updated 2019 Jun 6]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan.
  8. Aztreonam package labeling. Bristol Myers Squibb. Rev. Sept 2018. Accessed Sept. 15, 2021 at

Further information

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