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Common Side Effects from Antibiotics, and Allergies and Reactions

Medically reviewed by L. Anderson, PharmD. Last updated on Mar 5, 2017.

Common side effects | Should I stop | Allergies | For cold or flu | List of antibiotics and their side effects | More resources

What Are the Common Side Effects of Antibiotics?

All medications have side effects. When used appropriately, antibiotics are relatively safe with typically few side effects. However, some antibiotics are notorious for producing side effects that can be especially intolerable. An antibiotic side effect is defined as an unwanted reaction that occurs in addition to the desirable therapeutic action of the antibiotic. Like any drug, antibiotic side effects can occur and may interfere with the patient’s ability to tolerate and finish the course of medication.

Common side effects with antibiotics include:

  • Rash
  • Soft stools, diarrhea
  • Upset stomach
  • Fungal (yeast) infections like thrush

Contact your health care provider if you have any of these side effects while taking an antibiotic:

  • Severe allergic reaction that results in difficulty breathing, facial swelling (lips, tongue, throat, face)
  • Severe watery or bloody diarrhea or stomach cramps
  • Vaginal yeast infection with white discharge and severe itching
  • Mouth sores or white patches in your mouth or on your tongue

Side effects of antibiotics can range from mild allergic reactions to severe and debilitating adverse events. These side effects are extremely variable from patient to patient and from antibiotic to antibiotic. However, there are some common side effects that may occur within 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

Usually, antibiotic treatment should not be stopped without a health care provider’s approval; all medication should be finished. Stopping antibiotics early due to side effects may allow the infection to worsen and may lead to antibiotic resistance, making an antibiotic less effective. Even if the infection appears to have cleared up before all of the medication is gone, the full course of antibiotic treatment should always be completed unless you are told otherwise by your doctor.

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. A more severe allergic reaction called anaphylaxis is a medical emergency that requires immediate medical attention.

Anaphylactic reactions due to antibiotics may include:

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

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

Are Antibiotics Effective for a Cold or Flu?

Antibiotics are used to kill bacterial infections; they are not effective against viral infections, such as a cold or the flu, or 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; it might actually 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.

Table 1: List of Antibiotics and Their Side Effects

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. This is a comprehensive overview, but not a complete list, of common antibiotics or side effects that may occur.

Antibiotic Class Antibiotic Class Members Most Common Side Effects Additional Clinical Comments
List of Penicillins Penicillin, amoxicillin, amoxicillin-clavulanate, ampicillin, piperacillin-tazobactam, nafcillin, oxacillin Rash, diarrhea, abdominal pain, nausea/vomiting, drug fever, hypersensitivity (allergic) reactions If bloody stools, anaphylaxis, severe skin reaction, fever occur contact health care provider immediately; ampicillin may cause pseudomembranous colitis
List of Cephalosporins Cephalexin, cefaclor, cefuroxime, ceftibuten, cefdinir, cefixime,  ceftriaxone Rash, diarrhea, nausea/vomiting (rare), hypersensitivity (allergic) reactions, serum sickness, vaginal candidiasis Cross-hypersensitivity may occur in patients with documented penicillin allergy
List of Aminoglycosides Gentamicin, tobramycin, amikacin Renal (kidney) toxicity, ototoxicity (hearing loss), dizziness, nausea/vomiting, nystagmus Long term antibiotic or multiple treatment periods may lead to greater risk for ototoxicity, renal (kidney) toxicity
List of Carbapenems Meropenem, ertapenem, doripenem, imipenem-cilastatin Diarrhea, nausea/vomiting, headache, rash, liver toxicity, eosinophilia (elevated white blood cells) Hypersensitivity reactions reported with meropenem, imipenem in patients with penicillin allergy
List of Antituberculosis
Rifampin, rifabutin, isoniazid, pyrazinamide, ethambutol, dapsone Diarrhea, nausea/vomiting, anorexia, hemolytic anemia, liver toxicity, headache, peripheral neuropathy, dizziness, reddish-orange body fluids (rifampin, rifabutin only) Sides effects vary among agents, check individually; vitamin B6 (pyridoxine) may be taken to help prevent peripheral neuropathy with isoniazid
List of Glycopeptides Vancomycin, telavancin Vancomycin: red man syndrome (flushing, hypotension, itching); phlebitis; telavancin: taste alteration, nausea/vomiting, headache, dizziness IV infusion of vancomycin over 60 minutes may prevent red man syndrome
List of Macrolides (i.e., side effects of Z Pak antibiotic) Erythromycin, azithromycin (Z Pak), clarithromycin Abdominal pain, diarrhea, anorexia, nausea/vomiting, taste alterations (clarithromycin) High rate of gastrointestinal (stomach) side effects; do not crush, chew, break, open enteric-coated or delayed-release pill
List of Sulfonamides Trimethoprin-sulfamethoxazole, erythromycin-sulfisoxazole, sulfadiazine Nausea/vomiting, diarrhea, anorexia, abdominal pain, rash, photosensitivity, headache, dizziness Avoid prolonged sunlight exposure; use sunscreen, wear protective clothing; antibiotic cause of Stevens Johnson Syndrome, Toxic Epidermal Necrolysis
List of Tetracyclines Tetracycline, doxycycline, minocycline Nausea/vomiting, diarrhea, anorexia, abdominal pain, tooth discoloration in children < 8 years, liver toxicity Avoid prolonged sunlight exposure; use sunscreen, wear protective clothing
Quinolones (i.e., Cipro antibiotic side effects) Ciprofloxacin (Cipro), levofloxacin (Levaquin), moxifloxacin (Avelox), ofloxacin (Floxin) Nausea/vomiting, diarrhea, abdominal pain, headache, lethargy, insomnia, photosensitivity (can be severe) Avoid prolonged sunlight exposure; use sunscreen, wear protective clothing;
moxifloxacin associated with higher rates of side effects1; tendon rupture (rare) more common in age > 60, with corticosteroid use, in kidney, heart, lung translant recipients
List of Lincomycin derivatives Clindamycin, lincomycin Pseudomembranous colitis (may be severe), diarrhea, nausea/vomiting, rash, hypersensitivity, 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. dificile); consider use of less toxic agents
Miscellaneous Metronidazole Nausea/vomiting, dizziness, headache, vaginal candidiasis, metallic taste Avoid alcohol use during treatment and for up to 3 days after, combined use may lead to cramps, nausea/vomiting, flushing, headache; may discolor urine red-brown

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
  • Complications from intravenous (IV) use of antibiotics (phlebitis)

In one study, antibiotic side effects led to greater than 140,000 emergency department admissions per year in the United States. Roughly 50 percent of emergency visits were due to reactions to antibiotics in the penicillin class of drugs, and the other 50 percent were due to a wide variety of antibiotics used to treat many different types of infections. In this study, children less than one year of age were found to have the highest rate of antibiotic side effects.

  • Allergic Reactions, Anaphylaxis: Allergic reactions account for the most common type of side effect with antibiotics. In one study, it was estimated that over 142,000 emergency department visits per year were due to antibiotic adverse events, and approximately four-fifths 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.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 due to no explainable reason. Five to twenty-five percent of patients may develop antibiotic-associated diarrhea. 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 dificile. If the diarrhea is severe, bloody, or is accompanied by stomach cramps of 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., Z Pak), clarithromycin (Biaxin), erythromycin, and tetracycline.2 Probiotics such as Saccharomyces boulardii (Florastor) have been shown to be effective in helping to prevent antibiotic associated diarrhea in children and adults.4
  • 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.
  • 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 drugs, that result in serious 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.3
  • Injection site reactions or phlebitis: A reaction to an antibiotic can occur if the antibiotic is given intravenously in 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 at or 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. Bartlett JG. Clinical practice. Antibiotic-associated diarrhea. N Engl J Med 2002:346:334-9.

3. The Merck Manual for Health Care Professionals. Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN).

4. 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. Accessed online 3/6/2017 at

Further information

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