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What is the Difference Between Amoxicillin and Penicillin?

Tricia Christensen
Updated Mar 03, 2024
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The antibiotics amoxicillin and penicillin are close relatives. Amoxicillin is a derivative of penicillin, and it was first isolated in 1972, while penicillin was discovered in the 1920s and first saw significant use toward the end of World War II. Scientists developed drugs like amoxicillin because penicillin was not always useful in treating all infections, and the two drugs work in similar ways by inhibiting cell wall growth of bacteria. Each drug has similar action against a variety of bacteria, but they do have differences that are worth noting. Some of these differences include the way they are made, their effectiveness against certain bacteria, and their side effect profile.

One way that amoxicillin differs from penicillin is how it is made. Amoxicillin is semi-synthetic, which means it is a natural substance that is transformed by application of certain chemicals. Penicillin is a synthetic drug, meaning it is created out of several dynamic laboratory processes.

The way that amoxicillin and penicillin differ in terms of effectiveness is often contested. It is known that amoxicillin has a higher penetration rate and a longer half-life. It also treats more types of bacteria than does penicillin. Amoxicillin is called a moderate spectrum drug, while penicillin is called a narrow spectrum drug.

Oral preparations of amoxicillin are often as effective as injected forms of penicillin for a variety of conditions. For instance, some studies found that oral amoxicillin was just as effective as injected penicillin to treat children with bacterial pneumonia. Amoxicillin may work more effectively over a shorter time period than penicillin. The drug can also be combined with clavulanate to treat bacteria that produce beta-lactamase.

There are many side effects that amoxicillin and penicillin share. Both can upset the stomach, and are especially known for causing diarrhea. Amoxicillin is thought to cause more stomach upset problems than penicillin, but some researchers assert the opposite is true. Also, the rate at which people feel side effects varies by the individual, and it’s possible for patients to have fewer side effects than would be clinically predicted.

Amoxicillin and penicillin are both associated with a high rate of allergic reaction. It’s not uncommon for people to have an anaphylactic allergic reaction to one of these drugs, and people allergic to one medicine are urged to avoid the other. Anaphylaxis can be detected by symptoms such as difficulty breathing, rash or hives, swelling of the mouth, face, and tongue, and feelings of faintness, sense of doom, and nausea. If these symptoms arise when a patient is taking either medicine, he or she needs emergency medical care.

Ultimately, these substances are more alike than different. Amoxicillin has greater application and can treat more types of bacteria, but both drugs share approximately equal efficacy in a variety of infections. Amoxicillin tends to be prescribed more often than penicillin.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Tricia Christensen
By Tricia Christensen
With a Literature degree from Sonoma State University and years of experience as a The Health Board contributor, Tricia Christensen is based in Northern California and brings a wealth of knowledge and passion to her writing. Her wide-ranging interests include reading, writing, medicine, art, film, history, politics, ethics, and religion, all of which she incorporates into her informative articles. Tricia is currently working on her first novel.
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Tricia Christensen
Tricia Christensen
With a Literature degree from Sonoma State University and years of experience as a The Health Board contributor, Tricia...
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