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What Is the Connection between Ampicillin and Amoxicillin?

By Lee Johnson
Updated Feb 09, 2024
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Ampicillin and amoxicillin are members of the penicillin family of antibiotics, and as such there are a few important connections between them; most concern their similar chemistry and pharmacological profile. The precise action of each drug does vary slightly, but they both work by weakening the cell walls that surround bacteria and, in most cases, they’re more or less interchangeable. They’re both regularly used to treat infections of the inner ear and gonorrhea, for instance, and have been proven effective against E. coli, streptococci, and H. influenzae bacteria. The side effects tend to be about the same, too, and people with penicillin allergies usually react just as profoundly to each.

Understanding Antibiotic Classes

Antibiotics are some of the most powerful bacteria-fighting medications available, though there are many different variations. Most of the time these sorts of drugs are divided by type, known as class, based on how exactly they work and what they target; a further and more specific classification groups medications by family. Class deals primarily with chemical approach, whereas family more often involves specific characteristics and overlaps in approach.

Ampicillin and amoxicillin are both in the broad beta-lactam class, and within that class are in the penicillin family. Like most beta-lactam members, these drugs work best against bacteria known as “gram positive,” and do so by breaking down the cell walls of any cell marked as bacterial. Like most penicillins, they work primarily by deactivating certain enzymes to make the bacteria vulnerable to the body’s own immune defenses.

How They Work

In most cases, bacterial cells create a protective cell wall around themselves in order to survive within the human body The wall acts as a shield to protect them from the attacks of the white blood cells, which are the immune system’s primary means of identifying and eliminating intruders. Antibiotics such as ampicillin and amoxicillin break down this wall, which makes it easier for white blood cells to kill the bacteria.

The approach is slightly different. Amoxicillin breaks down the wall by disrupting its polymer chains, while ampicillin removes a basic component needed to build the wall generally. A simplified way to think about this using an analogy of a brick wall surrounding the bacteria is that amoxicillin disturbs the bricks in the wall and causes them to fall down, whereas ampicillin damages the bricks before the wall can be built at all.

Uses and Effectiveness

Different strains of bacteria require different antibiotics to treat them. There are a couple of reasons for this, but is largely because bacteria are not all susceptible to the same tricks employed by the drugs to break down the cell wall. In most parts of the world, medicines in the penicillin family are among the most prescribed, largely because they work against many of the most common bacterial problems. They also tend to be somewhat mild, and don’t have to be taken with many special precautions; food isn’t essential, for instance, and interval timing of doses matters, but isn’t crucial the way it is with drugs in other classes.

Both amoxicillin and ampicillin are suitable for use against E. coli, H. influenzae, streptococci, N. gonorrhea, and some strains of staphylococci. Ampicillin is also effective against salmonella, though amoxicillin isn’t usually as good of a choice for this particular ailment. Ampicillin is also used against more serious infections such as meningitis and endocarditis, and amoxicillin also works against pneumonia and bronchitis.

Side Effects and Common Precautions

Most members of the penicillin family have about the same side effects. Most commonly, these include vomiting, abdominal pain, and dizziness. Diarrhea and nausea have also been reported. Patients may also suffer allergic reactions with both of the treatments. The similarity between the actions of ampicillin and amoxicillin mean that many patients will be allergic to both treatments, and anyone who is allergic to penicillin should not take either drug.

As with any antibiotics, it’s important that people prescribed either preparation complete the entire course prescribed. It can tempting to stop taking pills once symptoms have subsided, but these sorts of medications keep working long after the body seems cured on its surface. Stopping midway through can actually make things worse in the long run, as it can allow bacterial cultures that remain to build up an immunity and flare back up even stronger than ever.

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Discussion Comments
By SteamLouis — On Aug 30, 2013

Yea, ampicillin and amoxicillin are very similar, they're both in the penicillin group. This is why I can't take them. I have a penicillin allergy so I can't use amoxicillin, ampicillin or augmentin. It's unfortunate because all of these drugs are very good and effective against many infections.

By ZipLine — On Aug 29, 2013

@burcidi-- I'm not a doctor or anything but I've taken both of these antibiotics. From what I understand, they're basically the same. They will work for mostly the same infections. I think both will be effective for a tooth abscess, although amoxicillin might be more commonly prescribed. Ampicillin is used more commonly in hospital setting but both of them are available in tablet form.

Have you seen a doctor? Let your doctor decide on the treatment.

By burcidi — On Aug 29, 2013

Which is better for treating a tooth abscess, ampicillin or amoxicillin?

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