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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:

More severe antibiotic side effects include:

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:

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:

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:

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*

Common Antibiotic Classes Antibiotic Class Drug Examples 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 aminoglycoside use 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. It is also used topically on the skin in Neosporin.

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, rifapentine only)

Multiple drugs are typically used to treat TB. Treatment usually lasts 6–12 months.

Isoniazid (INH) may be used as a first-line agent, and may be combined with rifampin, pyrazinamide, and ethambutol.

Sides effects vary among agents, so check each drug individually.

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

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

Other side effects in this drug class can include: abdominal pain, back pain, diarrhea, edema (fluid retention), flatulence (gas), foamy urine, itch, low potassium levels, rash, taste disturbance, urinary tract infection, vomiting.

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 (not approved in the U.S.), 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.

Non-antibiotic medicines may also be classified as "sulfa" drugs, such as carbonic anhydrase inhibitors, sulfonylureas, and thiazide, thiazide-like and loop diuretics.

List of tetracyclines

 

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

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

Used to treat infections caused by susceptible microorganisms such as gram+ and gram- bacteria, chlamydiae, mycoplasma, protozoans, or rickettsiae.

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

Due to antibacterial resistance within the tetracycline class, culture and susceptibility (C&S) testing is often recommended before beginning treatment.

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

Fluoroquinolones are considered broad-spectrum antibiotics and may be effective against a wide range of bacteria.

However, 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: includes tendon rupture, tendonitis, peripheral neuropathy, aggravation of myasthenia gravis, aortic aneurysm or dissection, low blood sugar, mental status changes.9

List of lincomycin derivatives
  • pseudomembranous colitis (may be severe and life-threatening)
  • diarrhea
  • nausea and vomiting
  • rash
  • allergies
  • jaundice (clindamycin)

Pseudomembranous colitis occurs in the colon and can cause severe inflammation, irritation and swelling. Clostridioides difficile (C. diff) is the main cause of pseudomembranous colitis. Symptoms of C. diff include: diarrhea, abdominal pain, and fever.

If severe diarrhea starts during treatment or for up to 8 weeks after treatment consult a health care provider immediately, as it may be pseudomembranous colitis (C. diff); consider use of less toxic agents.
List of nitroimidazoles
  • 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.

Semi-synthetic pleuromutilin antibiotic
  • lefamulin (Xenleta) for treatment of community-acquired bacterial pneumonia
  • oral tablets: diarrhea, nausea, vomiting, hepatic (liver) enzyme elevation
  • IV form: injection site reactions, liver enzyme elevation, nausea, low potassium levels, trouble sleeping, headache

Xenleta is a first-in-class, semi-synthetic pleuromutilin antibiotic.

Macrolides and fluoroquinolones (such as moxifloxacin) are options for treatment of community-acquired bacterial pneumonia (CABP) but have limitations due to resistance and safety concerns.

Xenleta has a short 5-to-7 day course of therapy (oral or IV).

*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

Select antibiotic side effect descriptions

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

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

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.

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.

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

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

Sources

  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, Kołodziej 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 https://packageinserts.bms.com/pi/pi_azactam.pdf
  9. Fluoroquinolone Antimicrobial Drugs Information. 6/18/2018. US Food and Drug Administration (FDA). Accessed Dec 11, 2024 at https://www.fda.gov/drugs/information-drug-class/fluoroquinolone-antimicrobial-drugs-information

Further information

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