Medically Reviewed on 2/22/2023

Generic Name: ampicillin

Brand Names: Omnipen, Polycillin, Principen

Drug Class: Penicillins, Amino

What is ampicillin, and what is it used for?

Ampicillin is an antibiotic used for treating infections of the 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.

  • Ampicillin belongs to a class of antibiotics called penicillins that are used for treating bacterial infections. Other members of this class include amoxicillin (Amoxil), piperacillin (Pipracil), ticarcillin (Ticar) and several others. These antibiotics all have a similar mechanism of action.
  • They stop bacteria from multiplying by preventing bacteria from forming the cell walls that surround them. The cell walls are necessary to protect bacteria from their environment and to keep the contents of the bacterial cell together.
  • Most bacteria cannot survive without a cell wall. Penicillins are most effective when bacteria are actively multiplying and forming cell walls. Ampicillin is effective against many bacteria including H. influenzae, N. gonorrhoea, E. coli, Salmonella, and Shigella, streptococci and certain strains of staphylococci.

Because of the increased use of antibiotics, many bacterial strains have become resistant to penicillin. Bacteria causing serious infections should be tested for resistance against penicillin and other antibiotics.

What are the side effects of ampicillin?

Common side effects of ampicillin include:

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 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. For persons who experience nausea or stomach distress after taking ampicillin, it may be taken with meals.

Which drugs interact with 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.

Pregnancy and breastfeeding

  • 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.


Bowel regularity means a bowel movement every day. See Answer

What else should I know about ampicillin?

What preparations of ampicillin are available?
  • Capsules: 250 and 500 mg. Powder oral suspension: 125 and 250 mg/5mL. Powder for injection: 250 mg, 500 mg, 1g, and 2 g.
How should I keep ampicillin stored?
  • Capsules and powder should be kept at room temperature from 15 C to 30 C (59 F to 86 F). After mixing the powder with water, it can be used for up to seven days if stored at room temperature or 14 days if refrigerated. It must be shaken before each use and should be kept well-sealed.
When was ampicillin approved by the FDA?
  • Ampicillin was approved by the FDA in 1963.


Ampicillin is an antibiotic used to treat infections caused by H. influenzae, N. gonorrhoea, E. coli, salmonella, shigella, streptococci, and certain strains of staphylococci. Common side effects of ampicillin include nausea, vomiting, loss of appetite, diarrhea, abdominal pain, rash, itching, headache, confusion and dizziness.

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See more info: ampicillin on RxList
Medically Reviewed on 2/22/2023
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