Ampicillin is a penicillin-type antibiotic used to treat a wide range of bacterial infections, including meningitis, urinary tract infections, respiratory infections, and gastrointestinal infections. It works by disrupting the construction of bacterial cell walls, which causes the bacteria to break apart and die. First introduced in the 1960s as a broader-spectrum alternative to earlier penicillins, it remains a frontline treatment for several serious infections today.
Infections Ampicillin Treats
Ampicillin is prescribed for infections in many parts of the body. Its approved uses include infections of the throat, sinuses, and lungs, as well as urinary tract infections, gastrointestinal infections (involving the stomach or intestines), and reproductive organ infections. It’s also one of the primary antibiotics used for bacterial meningitis, an infection of the protective layers surrounding the brain and spinal cord.
What makes ampicillin especially valuable is its effectiveness against certain bacteria that other penicillins can’t reliably handle. It covers a mix of gram-positive bacteria (like streptococci, enterococci, and Listeria) and gram-negative bacteria (like E. coli, Salmonella, Shigella, and the bacteria responsible for gonorrhea and meningococcal disease). That dual coverage is the reason it shows up so often in hospital settings where doctors need to act before lab results confirm exactly which bacterium is causing the problem.
A Critical Role in Newborn Care
One of ampicillin’s most important uses is in treating suspected sepsis in newborns. Sepsis, a life-threatening bloodstream infection, affects millions of newborns worldwide and causes roughly 200,000 neonatal deaths each year, many of which are preventable with early antibiotic treatment. The World Health Organization recommends ampicillin (paired with another antibiotic called gentamicin) as the first-choice regimen for suspected sepsis in hospitalized infants, typically given intravenously for 7 to 10 days.
This combination works because ampicillin covers Listeria and enterococci, two bacteria that are particularly dangerous in newborns and that many other antibiotics miss. Gentamicin fills in the gaps against additional gram-negative organisms. Together, they provide broad enough coverage to protect infants while doctors wait for culture results to identify the specific pathogen.
How Ampicillin Is Given
Ampicillin comes in both oral (capsule) and injectable forms, but the two aren’t interchangeable in practice. When taken by mouth, ampicillin has relatively low absorption, with only about 20% of the drug making it into the bloodstream. That’s roughly half the oral absorption of amoxicillin, a closely related antibiotic. This is the main reason doctors typically prefer amoxicillin for infections that can be treated with pills at home.
For more serious infections like meningitis, sepsis, or severe urinary tract infections, ampicillin is given intravenously or as an intramuscular injection. The injectable form bypasses the absorption problem entirely, delivering the full dose directly into the bloodstream. If you’re prescribed the oral form, taking it on an empty stomach helps improve absorption.
Why Ampicillin Over Amoxicillin?
Since amoxicillin absorbs better and is generally easier to take, you might wonder why ampicillin still exists. The answer comes down to specific clinical situations. Ampicillin is preferred for IV treatment of serious infections like meningitis and endocarditis (infection of the heart valves), where injectable antibiotics are necessary. It’s also the go-to choice for Listeria infections, which can be dangerous for newborns, pregnant women, and people with weakened immune systems. In many hospitals, ampicillin is simply the standard of care for these particular conditions because decades of clinical use have confirmed it works.
Bacteria That Resist Ampicillin
Ampicillin doesn’t work against every bacterium it once did. Some bacteria produce enzymes called beta-lactamases that break down the drug before it can do its job. Staphylococci that produce these enzymes are already resistant, which is why the FDA notes ampicillin only works against “nonpenicillinase-producing” staph strains. More concerning are extended-spectrum beta-lactamase (ESBL) producing bacteria, particularly certain strains of E. coli and Klebsiella pneumoniae, which resist not just ampicillin but many related antibiotics as well. The CDC considers ESBL-producing bacteria a significant public health threat because they can make common infections much harder to treat.
This growing resistance is one reason ampicillin isn’t used as a first-line antibiotic for routine infections the way it once was. When doctors suspect an infection might involve resistant bacteria, they’ll often choose a different antibiotic or wait for lab results showing which drugs the specific bacteria respond to.
Common Side Effects
The most frequent side effects are digestive: nausea, vomiting, and diarrhea. These happen because the drug doesn’t just kill the bacteria causing your infection; it also disrupts some of the normal bacteria in your gut. Mild diarrhea is common enough that it’s generally not a reason to stop the medication, but diarrhea lasting more than two days or that becomes severe and watery can signal a more serious problem called C. difficile infection, which needs prompt attention.
Skin rashes are another well-known side effect. Some are simple allergic reactions with itching and hives, while others are more serious. Rarely, ampicillin can trigger severe skin reactions that involve blistering, peeling, or redness spreading across the body, sometimes appearing weeks to months after starting the drug. Swelling of the face, lips, tongue, or throat is a sign of a serious allergic reaction that needs immediate medical care. People with a known penicillin allergy should not take ampicillin, since it belongs to the same drug family.
Mouth sores, throat irritation, and vaginal yeast infections (caused by the disruption of normal bacteria) are also reported but tend to be less common.
Safety During Pregnancy
Ampicillin is considered one of the safer antibiotics to use during pregnancy. The Mayo Clinic lists it alongside amoxicillin as an antibiotic generally thought to be safe for pregnant women. It’s frequently used during labor to prevent group B streptococcus transmission from mother to baby, a routine practice in many hospitals. As with any medication during pregnancy, the choice to use ampicillin involves weighing the risk of the infection against the safety profile of the drug, but ampicillin has a long track record of use in this population without evidence of harm to the developing baby.