Amoxicillin vs. Ampicillin: What's the difference?

What is amoxicillin? What is ampicillin?

Amoxicillin is a penicillin-type antibiotic. Other members of this class include ampicillin (Unasyn), piperacillin (Pipracil), and ticarcillin (Ticar). These antibiotics do not directly kill bacteria, but they stop bacteria from multiplying by preventing bacteria from forming the walls that surround them. The walls are necessary to protect bacteria from their environment and to keep the contents of the bacterial cell together. Bacteria cannot survive without a cell wall. Amoxicillin is effective against many different bacteria including H. influenzae, N. gonorrhoea, E. coli, Pneumococci, Streptococci, and certain strains of Staphylococci. Amoxicillin is used to treat bacterial infections of the middle ear, tonsils, throat, larynx (laryngitis), bronchi (bronchitis), lungs (pneumonia), urinary tract, and skin. It also is used to treat gonorrhea.

Ampicillin is a penicillin-type antibiotic used for treating bacterial infections middle ear, sinuses, stomach and intestines, bladder, and kidney caused by susceptible bacteria. It also is used for treating uncomplicated gonorrhea, meningitis, endocarditis and other serious infections. Other members of the same drug class include amoxicillin (Amoxil), piperacillin (Pipracil), and ticarcillin (Ticar). These antibiotics stop bacteria from multiplying by preventing bacteria from forming the cell walls that surround them. Ampicillin is effective against many bacteria including H. influenzae, N. gonorrhoea, E. coli, Salmonella, and Shigella, streptococci and certain strains of staphylococci.

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What are the side effects of amoxicillin and ampicillin?

Amoxicillin

Side effects due to amoxicillin include

People who are allergic to the cephalosporin class of antibiotics, which are related to the penicillins, for example, cefaclor (Ceclor), cephalexin (Keflex), and cefprozil (Cefzil), may or may not be allergic to penicillins.

Serious but rare reactions include:

Amoxicillin can alter the normal bacteria in the colon and encourage overgrowth of some bacteria such as Clostridium difficile which causes inflammation of the colon (pseudomembranous colitis). Patients who develop signs of pseudomembranous colitis after starting amoxicillin (diarrhea, fever, abdominal pain, and possibly shock) should contact their physician immediately.

Ampicillin

Common side effects of ampicillin include:

  • nausea,
  • vomiting,
  • loss of appetite,
  • diarrhea,
  • abdominal pain,
  • rash,
  • itching,
  • headache,
  • confusion and
  • dizziness.

Patients with a history of allergic reactions to other penicillins should not receive ampicillin. Persons who are allergic to the cephalosporin class of antibiotics, which are related to the penicillins may or may not be allergic to penicillins [for example, cefaclor (Ceclor), cephalexin (Keflex), and cefprozil (Cefzil)]. Serious but rare reactions include seizures, severe allergic reactions (anaphylaxis), and low platelet or red blood cell count.

Ampicillin can alter the normal bacteria in the colon and encourage overgrowth of some bacteria such as Clostridium difficile which causes inflammation of the colon (pseudomembranous colitis). Patients who develop signs of pseudomembranous colitis after starting ampicillin (diarrhea, fever, abdominal pain, and possibly shock) should contact their physician immediately.

What is the dosage for amoxicillin vs. ampicillin?

Amoxicillin

  • For most infections in adults the dose of amoxicillin is 250 mg every 8 hours, 500 mg every 8 hours, 500 mg every 12 hours or 875 mg every 12 hours, depending on the type and severity of infection.
  • For the treatment of adults with gonorrhea, the dose is 3 g given as one dose.
  • For most infections, children older than 3 months but less than 40 kg are treated with 25 or 45 mg/kg/day in divided doses every 12 hours or 20 or 40 mg/kg/day with one-third of the daily dose given every 8 hours depending on the type and severity of the infection.
  • Amoxicillin can be taken with or without food.

Ampicillin

  • The usual oral dose range for most infections is 250 to 500 mg 4 times daily for 7-14 days. Injectable doses range from 250 to 2000 mg 4 times daily. When used to treat gonorrhea, a single 3.5 gram dose (seven 500 mg capsules) is administered with 1 g probenecid (Benemid).
  • The probenecid slows down the elimination of ampicillin so that ampicillin remains in the body longer. Food in the stomach reduces how much and how quickly ampicillin is absorbed.
  • Therefore, ampicillin should be taken either 1 hour prior to or 2 hours following a meal for maximal absorption; however, for persons who experience nausea or stomach distress after taking ampicillin, it may be taken with meals.

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What drugs interact with amoxicillin and ampicillin?

Amoxicillin

Amoxicillin is rarely associated with important drug interactions.

Ampicillin

Probenecid (Benemid) causes an increase in the amount of ampicillin in the body. Use of ampicillin with allopurinol (Zyloprim) can increase the incidence of drug-related skin rash. Ampicillin may reduce the effect BCG live vaccine and Typhoid live vaccine.

Are amoxicillin and ampicillin safe to use while pregnant of breastfeeding?

Amoxicillin

  • Penicillins are generally considered safe for use by pregnant women who are not allergic to penicillin.
  • Small amounts of amoxicillin may be excreted in breast milk and may cause diarrhea or allergic responses in nursing infants. Amoxicillin is generally considered safe to use while breastfeeding. Amoxicillin is used to treat infections in the newborn.

Ampicillin

  • Ampicillin is considered safe during pregnancy.
  • Ampicillin is excreted in breast milk and may cause diarrhea or allergic responses in nursing infants. If ampicillin is used during pregnancy, the potential benefit of ampicillin for the mother should be weighed against the potential risk of side effects in the infant. Ampicillin is used for treating infants.

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Summary

Amoxicillin (Moxatag) and ampicillin (Amoxil) are penicillin-type antibiotics used to treat bacterial infections of the middle ear and respiratory tract and to treat gonorrhea. Amoxicillin is also used to treat bacterial infections of the tonsils, throat, larynx (laryngitis), urinary tract, and skin. Ampicillin is also used to treat bacterial infections of the sinuses, stomach and intestines, bladder, and kidney. Ampicillin is also used to treat meningitis, endocarditis, and other serious infections.

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Medically Reviewed on 5/29/2019
References
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