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Ampicillin

Antibiotic

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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.
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Possible side effects

  • 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.

Symptoms of an allergic reaction include: rash, hives, itching, chills, fever, headache, muscle ache, shortness of breath, coughing, tightness in the throat, swelling of the face or neck.

Not all patients who take ampicillin will experience these side effects. Common side effects are bolded, but there may be others. Please report all suspected side effects to your doctor or pharmacist.

Find more information on side effects.

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Tips for patients and families

Be sure to discuss all questions and instructions with your doctor or pharmacist. 

  • 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.
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Always check the medication label. Many medicines look the same. Do not take any new medicine, including over-the counter medicines or supplements, without checking with your doctor or pharmacist. Talk to your doctor or pharmacist about what to do in the case of a missed dose, allergic reaction, or medical emergency. 

This information is not intended to be a complete guide and is not a substitute for talking with your provider. Medicine names, appearance, formulas, or availability may vary by clinic location. Always follow the instructions given by your care team.

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