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What is ampicillin?

Ampicillin (also called Omnipen® or Principen®) is used to prevent or treat infections. It is available in the following forms and strengths, all of which are taken by mouth:

  • 250-mg capsule
  • 500-mg capsule
  • 50-mg/mL liquid suspension

Ampicillin is also available as a clear liquid given by vein (IV).

How to take and store ampicillin

  • You should take ampicillin on an empty stomach, 1 hour before or 2 hours after meals.
  • Shake the liquid suspension well before you take it. Use a medicine cup or oral syringe to measure your dose to be sure you are taking the correct amount of medicine.
  • Store the liquid suspension in the refrigerator.
  • Store the capsules at room temperature in a dry place.
  • Discard the liquid suspension after 14 days or when the expiration date on the label tells you to do so.

Possible side effects of ampicillin

  • Nausea, vomiting
  • Diarrhea
  • Rash
  • Loss of appetite
  • Headache
  • Stomach pain
  • Allergic reaction with the following symptoms: rash, hives, itching, chills, fever, shortness of breath, muscle aches, swelling of face or neck, tightness in throat, or cough.

These are the most common side effects, but there may be others. Please report all side effects to the doctor or nurse.

In case of a severe side effect or reaction, call 595-3300. If you are outside the Memphis area, dial toll-free 1-866-2STJUDE (1-866-278-5833), and press 0 once the call is connected.

Special instructions for ampicillin

  • Before taking ampicillin, tell your doctor if you have had an allergic reaction to any other penicillin. If you experience any of the allergic symptoms (listed under “Possible side effects” or if you have severe or watery diarrhea, tell your doctor right away.
  • Take all doses of this medicine as instructed by your doctor, even if you begin to feel better before all doses are taken.
  • Tell your doctor if you are taking probenecid or methotrexate, and do not take ampicillin with these medicines unless your doctor tells you to do so.
    • Probenecid may increase the levels of ampicillin in your body.
    • Ampicillin may increase the levels of methotrexate in your body.
  • Birth control medicines that contain estrogen may not be effective while you are taking ampicillin. Use a second birth control method for at least 1 month after taking ampicillin.
  • Ampicillin may cause a false-positive urine test result when your urine is checked for sugar.


This document is not intended to take the place of the care and attention of your personal physician or other professional medical services. Our aim is to promote active participation in your care and treatment by providing information and education. Questions about individual health concerns or specific treatment options should be discussed with your physician.

St. Jude complies with health care-related federal civil rights laws and does not discriminate on the basis of race, color, national origin, age, disability, or sex.

ATTENTION: If you speak another language, assistance services, free of charge, are available to you. Call 1-866-278-5833 (TTY: 1-901-595-1040).

ATENCIÓN: si habla español, tiene a su disposición servicios gratuitos de asistencia lingüística. Llame al 1-866-278-5833 (TTY: 1-901-595-1040).

تنبيه: إذا كنت تتحدث باللغة العربية فيمكنك الاستعانة بخدمات المساعدة اللغوية المتوفرة لك مجانا. .يرجى الاتصال بالرقم. 5833-278-866-1  (الهاتف النصي: 1040-595-901-1).