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Overview: Ampicillin is an antibiotic that is used to treat bacterial infections. It kills certain bacteria by preventing them from being able to create their cell wall. Common side effects include diarrhea, nausea, vomiting, or rash. Serious side effects are rare but may include severe diarrhea or allergic reactions. Ampicillin is available as an oral capsule and an injection.

Uses

What is ampicillin used for?

Ampicillin is an antibiotic that is commonly used for infections caused by certain bacteria.

Ampicillin may not be appropriate for all infections, depending on which germs are causing the infection. Some bacteria may have developed resistance to ampicillin, which will make it not work as well. 

Ampicillin only treats certain infections caused by bacteria. Ampicillin will not work for infections caused by viruses, such as the common cold, the flu, or coronavirus. It also will not work for any other infections that are not caused by bacteria. Using ampicillin or any other antibiotic when it is not needed may cause the antibiotic to not work for bacterial infections in the future.

Ampicillin may also be used for other conditions as determined by your healthcare provider.

How does ampicillin work (mechanism of action)?

Ampicillin kills certain bacteria by preventing them from being able to create their cell wall.

How is ampicillin supplied (dosage forms)?

Brands and other names

  • Amcill
  • Omnipen
  • Penbritin
  • Polycillin
  • Principen
  • Totacillin

Dosage forms and strengths

  • 250 mg oral capsules
  • 500 mg oral capsules

Ampicillin is also available in injectable forms.

How should I store ampicillin?

Ampicillin should be stored at room temperature, between 68 F to 77 F (20 C to 25 C). It can be exposed to temperatures between 59 F to 86 F (15 C to 30 C) for shorter periods of time, such as when transporting it. Store in a cool, dry place.

Side Effects

What are the most common side effects of ampicillin?

The most common side effects of ampicillin are listed below. Tell your healthcare provider if you have any of these side effects that bother you.

  • Diarrhea (see below)
  • Nausea and vomiting (when taken by mouth)
  • Skin rash
  • Inflamed or sore mouth, lips, or tongue (when taken by mouth)

There may be other side effects of ampicillin that are not listed here. Contact your healthcare provider if you think you are having a side effect of a medicine. In the U.S., you can report side effects to the FDA at www.fda.gov/medwatch or by calling 800-FDA-1088. In Canada, you can report side effects to Health Canada at www.health.gc.ca/medeffect or by calling 866-234-2345.

What are the serious side effects of ampicillin?

While less common, the most serious side effects of ampicillin are described below, along with what to do if they happen.

Severe Allergic Reactions. Ampicillin may cause allergic reactions, which can be serious. Stop using ampicillin and get help right away if you have any of the following symptoms of a serious allergic reaction.

  • Breathing problems or wheezing
  • Racing heart
  • Fever or general ill feeling
  • Swollen lymph nodes
  • Swelling of the face, lips, mouth, tongue, or throat
  • Trouble swallowing or throat tightness
  • Itching, skin rash, or pale red bumps on the skin called hives
  • Nausea or vomiting
  • Dizziness, feeling lightheaded, or fainting
  • Stomach cramps
  • Joint pain

Antibiotic-associated Diarrhea. An overgrowth of germs called Clostridioides difficile, or “C. diff,” can happen in your gut with many types of antibiotics, including ampicillin. This can cause a condition known as antibiotic-associated diarrhea or pseudomembranous colitis, and can happen even up to two months after stopping the antibiotic. Call your healthcare provider right away if you have any of the following symptoms of this condition.

  • Watery diarrhea
  • Diarrhea that does not go away
  • Blood in your stool (poop)
  • Severe stomach cramps
  • Fever that starts after you start or finish the medicine

Blood Disorders. Ampicillin can cause blood disorders, such as agranulocytosis, anemia, low platelet levels (thrombocytopenia), and low white blood cell levels (leukopenia). Stop using ampicillin and get help right away if you have any of the following symptoms of blood disorders.

  • Fever that does not go away
  • Shortness of breath
  • Pale or yellowish skin
  • Easy bruising or bleeding
  • Frequent infection
  • Unusual weakness or tiredness
  • Dizziness, lightheadedness, or feeling like you are about to pass out
  • Headache
  • Fast or abnormal heartbeat

Seizures. Some people who have used ampicillin have reported having seizures. There are many different types of seizures and some of them are not easy to recognize. Stop using ampicillin and call your healthcare provider if you have any of the following signs that a seizure is happening or could happen.

  • Muscle twitching, jerking, spasms, tremors, or other uncontrolled movements
  • Stiffening of your arms and legs
  • Feeling anxious, nervous, confused, or a sense of dread
  • Feeling dizzy or lightheaded
  • Falling or losing consciousness

Warnings & Precautions

Who should not use ampicillin?

Allergies to Ingredients. People who are allergic to any of the following should not use ampicillin.

  • Ampicillin
  • Any of the ingredients in the specific product dispensed

Your pharmacist can tell you all of the ingredients in the specific ampicillin products they stock.

Allergies to Certain Antibiotics. Some people who have an allergy to certain other antibiotics, such as penicillin, may also be allergic to ampicillin. Let your healthcare provider know if you have ever had a severe allergic reaction to an antibiotic.

What should I know about ampicillin before using it?

Do not use ampicillin unless it has been prescribed to you by a healthcare provider. Use it as prescribed.

Do not share ampicillin with other people, even if they have the same condition as you. It may harm them.

Keep ampicillin out of the reach of children.

Ampicillin capsules should be taken on an empty stomach with a full glass of water. 

What should I tell my healthcare provider before using ampicillin?

Tell your healthcare provider about all of your health conditions and any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. This will help them determine if ampicillin is right for you.

In particular, make sure that you discuss any of the following.

Current and Past Health Conditions. Tell your healthcare provider if you have any of the following.

  • An allergy to penicillin or any other medicines
  • Liver problems
  • Kidney problems
  • History of antibiotic-associated diarrhea
  • Mononucleosis, also called “mono” or the kissing disease

Other Medicines and Supplements. Ampicillin may interact with other medicines and supplements. Before using ampicillin, tell your healthcare provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using. See the Interactions section for more details.

Pregnancy. It is not known if or how ampicillin could affect pregnancy or harm an unborn baby. Tell your healthcare provider if you are or plan to become pregnant. Your healthcare provider will advise you if you should use ampicillin while you are pregnant or trying to get pregnant. 

Breastfeeding. Ampicillin passes into breast milk. Tell your healthcare provider if you are breastfeeding or plan to breastfeed. Your healthcare provider will advise you if you should use ampicillin while breastfeeding.

Interactions

Does ampicillin interact with foods or drinks?

Food can lower the levels of ampicillin in your body. Do not take ampicillin capsules within 30 minutes before meals or within two hours after meals.

It is unknown if drinking alcohol will affect ampicillin, but heavy drinking can slow your immune response and make it harder for your body to fight infection.

Does ampicillin interact with other medicines (drug interactions)?

Always tell your healthcare provider about any prescription or over-the-counter (OTC) medicines, vitamins/minerals, herbal products, and other supplements you are using.

In particular, make sure that you discuss if you are using any of the following before taking ampicillin.

  • A birth control pill
  • Allopurinol or probenecid, which is a medicine commonly used for gout
  • A macrolide antibiotic, such as clarithromycin or erythromycin (Ery-Tab and others), which is a medicine used for bacterial infections
  • A tetracycline, such as tetracycline, doxycycline, minocycline, omadacycline (Nuzyra), or sarecycline (Seysara), which is a medicine used to treat infections or other conditions
  • Any other antibiotic, which is a medicine used for bacterial infections

This may not be a complete list of medicines that can interact with ampicillin. Always check with your healthcare provider.

Does interact with other drugs you are taking?

Enter your medication into the WebMD interaction checker

Overdose/Missed Dose

What should I do if I accidentally use too much ampicillin?

If you or someone else has used too much ampicillin, get medical help right away, call 911, or contact a Poison Control center at 800-222-1222.

What should I do if I miss a dose of ampicillin?

If you miss a dose, take it as soon as you remember. If it is almost time for your next dose, skip the missed dose and only take the next dose. Do not take double or extra doses.

It is important to complete your prescribed course of ampicillin, even if your symptoms get better or go away. This will reduce the risk that future infections are resistant to ampicillin or other similar medicines.