What's the Difference Between Penicillin and Amoxicillin?

Medically Reviewed by Nayana Ambardekar, MD on March 10, 2024
2 min read

Amoxicillin and penicillin are both antibiotics in the penicillin class. Alexander Fleming first discovered penicillin in 1928, and it was available to treat infections by the 1940s. Before this time, there were no antibiotics to treat the infections which are easily cured today thanks to his discoveries.

Amoxicillin is a semi-synthetic penicillin that became available in 1972. Amoxicillin is available in the U.S. under the brand name Amoxil.

Both are used to treat bacterial infections. They will not work on viral infections like a cold, the flu, or COVID-19

Penicillin. Your doctor may prescribe penicillin for things like strep throat, bacterial pneumonia, bacterial meningitis, and the bacteria that causes syphilis. There are many other bacterial infections that penicillin works against.

Doctors used to prescribe penicillin to treat gonorrhea and staph infections, but it is no longer recommended because of increasing bacterial resistance to the antibiotic.

Amoxicillin. Amoxicillin is prescribed for oral (mouth) infections, ear infections, and pneumonia, among other bacterial infections. It can also work against sinus infections and other lung infections like bronchitis

Amoxicillin used to be a primary choice of antibiotic for urinary tract infections, the majority of which are caused by the bacteria E. coli. However, it is not recommended because of antibiotic resistance.

Both of these antibiotics have similar side effects that go away when you stop taking the medication. They include:

Less common side effects include:

  • Severe diarrhea
  • Black or bloody stool
  • Stomach cramps
  • Seizures
  • Nausea
  • Vomiting
  • Depression
  • Sore throat
  • Fever
  • Bruising easily
  • Yellow eyes and skin
  • Hallucinations
  • Confusion

Call your doctor if you have any unusual side effects.

Let your health care provider know if you’ve had an allergic reaction in the past to penicillin or amoxicillin.

Signs of an allergic reaction to either of these drugs may include:

The dosage of each medication may depend on the condition you are treating. When taking antibiotics, always take the full regimen prescribed by your doctor, even when you start feeling better. Taking all the medication helps to avoid antibiotic resistance in the future.

Both penicillin and amoxicillin are considered safe to use during pregnancy and while breastfeeding. However, your doctor will only prescribe a drug during pregnancy or breastfeeding when the benefits will be greater than the risks.