Merck Manual

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Brian J. Werth

, PharmD, University of Washington School of Pharmacy

Last full review/revision Jul 2020| Content last modified Jul 2020
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Penicillins are a subclass of antibiotics called beta-lactam antibiotics (antibiotics that have a chemical structure called a beta-lactam ring). Carbapenems, cephalosporins, and monobactams are also beta-lactam antibiotics.

Penicillins are used to treat infections caused by gram-positive bacteria (such as streptococcal infections) and some gram-negative bacteria (such as meningococcal infections).

Penicillins include the following:

  • Amoxicillin

  • Ampicillin

  • Carbenicillin

  • Dicloxacillin

  • Nafcillin

  • Oxacillin

  • Penicillin G

  • Penicillin V

  • Piperacillin

  • Ticarcillin

Most bacteria have an outer covering (cell wall) that protects them. Like the other beta-lactam antibiotics, penicillins work by preventing bacteria from forming this cell wall, resulting in death of the bacteria.

Some bacteria produce enzymes that can inactivate beta-lactam antibiotics. For infections caused by these bacteria, penicillins are given with a drug that can inhibit these enzymes, such as clavulanate or sulbactam. Common combinations include the following:

  • Ampicillin plus sulbactam

  • Amoxicillin plus clavulanate

  • Piperacillin plus tazobactam

  • Ticarcillin plus clavulanate

Some penicillins can be given by mouth (for example, amoxicillin and penicillin V) or by injection (for example, piperacillin). Others (such as ampicillin) can be given either way.

Food does not interfere with the absorption of amoxicillin, but penicillin G should be taken 1 hour before or 2 hours after a meal. Amoxicillin tends to be used more often than ampicillin (when taken by mouth) because amoxicillin is absorbed into the bloodstream better, has fewer gastrointestinal side effects, and can be given less frequently.




Common Uses

Some Side Effects









Penicillin G

Penicillin V



Wide range of infections, including streptococcal infections, enterococcal infections, syphilis, and Lyme disease

Dicloxacillin, nafcillin, and oxacillin: Most often used to treat staphylococcal infections

Nausea, vomiting, and diarrhea*

Brain and kidney damage (rare)

* Almost any antibiotic can cause Clostridioides difficile–induced diarrhea and inflammation of the colon (colitis), but clindamycin, penicillins, cephalosporins, and fluoroquinolones are the most common causes.

Use of Penicillins During Pregnancy and Breastfeeding

Penicillins are among the safest antibiotics to use during pregnancy. However, they are used only when the benefits of treatment outweigh the risks. (See also Drug Use During Pregnancy.)

Use of penicillins during breastfeeding is generally considered acceptable. (See also Drug Use During Breastfeeding.)

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