What Is the Difference Between Penicillin & Amoxicillin?

Penicillin and amoxicillin are more alike than different, but there are minor differences between these drugs, including indications for use, form and dosing.

Penicillin and amoxicillin are commonly prescribed medications used to treat infections or diseases caused by bacteria. Both fall within the same family of drugs, making them similar in many ways. These medications are both considered broad spectrum antibiotics, which means they treat a wide array of infectious diseases. But there are some differences between these drugs, particularly in their indications for use, form, cost and dosing.

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While both penicillin and amoxicillin are used to treat bacterial infections, a doctor may prescribe one over the other depending on the type of infection that needs treatment. Both can be effective against upper respiratory or skin infections, although amoxicillin, for instance, may be the preferred drug for some other conditions, such as ear or urinary tract infections. There are many antibiotics available for use, and the choice of drug is not only based on known benefits but can also be determined by laboratory testing of blood, sputum or urine to determine which antibiotic will work best to treat the infection.

Form and Cost

While both of these antibiotics are derived from a mold, penicillin is synthetically produced, while amoxicillin is a semi-synthetic penicillin-based drug. Both types of drugs come in several forms, including liquid and tablets. Amoxicillin and penicillin are both available as lower cost generic drugs, however if your doctor prescribes a brand-name version, your cost may be higher, depending on your health insurance coverage.


According to the Merck Manual, amoxicillin and penicillin differ in how they interact with food in the stomach. Amoxicillin can be taken with food, while for best absorption, penicillin needs to be taken 1 hour before or 2 hours after a meal. Because there are many different forms of these antibiotics, it can be difficult to compare dosing effectiveness. While the antibiotic prescribed is usually personalized based on the type of infection and health status, the specific dose and the number of days the drug is needed may also be based on age, health status and body size.


Amoxicillin and penicillin are more alike than they are different. Amoxicillin tends to be used more commonly since it can treat more types of bacteria, but both drugs are considered effective treating a variety of infections. Regardless of the antibiotic prescribed, it's important to take all the pills as directed, as stopping treatment early can result in a resurgent, more difficult to treat infection. Be sure to let your doctor know if you are pregnant, plan to become pregnant or are breastfeeding, so the safest antibiotic can be chosen for you. Also, let your doctor know if you have any side effects such as a rash, stomach pain or diarrhea. If you have severe side effects, such as severe or bloody diarrhea, or signs of an allergic reaction such as severe hives, difficulty breathing or any other new or unusual symptoms, seek immediate medical care.

Reviewed by Kay Peck, MPH RD