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What's the difference between amoxicillin and penicillin?

Medically reviewed by C. Fookes, BPharm. Last updated on Sep 3, 2019.

Official Answer

by Drugs.com

The main difference between amoxicillin and penicillin is that amoxicillin is effective against a wider spectrum of bacteria compared with penicillin. Both amoxicillin and penicillin belong to the class of antibiotics called penicillins.

Other differences between amoxicillin and penicillin include:

  • Brand names of amoxicillin include Moxatag and Amoxil
  • Brand names of penicillin G include Permapen Isoject and Pfizerpen 
  • Although side effects of amoxicillin and penicillin are similar, amoxicillin is more likely to cause a rash, which is not necessarily the sign of an allergy
  • Amoxicillin may be used to treat genitourinary infections, acute, uncomplicated gonorrhea, and H. pylori eradication in combination with other medications  
  • Penicillin may be used to treat or prevent Rheumatic fever or chorea, bacterial endocarditis, bacterial meningitis, or dental infections involving gum tissue
  • Penicillin was discovered in 1928 and came into widespread use during World War II. Amoxicillin was discovered in 1958 and came into medical use in 1972. 

Because both amoxicillin and penicillin are penicillins, they have many similarities, for example: ·

  • Both may be considered if antibiotics are necessary to treat otitis media, respiratory tract infections, or skin and soft tissue infections caused by susceptible bacteria
  • Both have a similar incidence of side effects, such as abdominal pain, diarrhea, heartburn, nausea, or vomiting
  • Both are available as generics. 

What is amoxicillin?

Amoxicillin is considered a broad-spectrum antibiotic, and is effective against the following organisms:

  • b-lactamase-negative) strains of Streptococcus spp. (a- and b-hemolytic strains only, such as S. pyrogenes)
  • S. pneumoniae
  • Non-penicillinase producing Staphylococcus spp.
  • H. influenzae
  • Helicobacter pylori
  • E. coli
  • P. mirabilis
  • E. faecalis
  • N. gonorrhoeae

Amoxicillin is available combined with clavulanic acid to give it activity against beta-lactamases.

The activity of amoxicillin is similar to ampicillin except that amoxicillin is resistant to gastric acid which improves its absorption and blood levels. Amoxicillin is approved to treat: 

  • Ear, nose, and throat infections (pharyngitis/tonsillitis, otitis media)
  • Acute, uncomplicated gonorrhea (anogenital and urethral infections) due to N. gonorrhoeae (males and females)
  • Genitourinary tract infections
  • Helicobacter pylori eradication (in combination with other medications 
  • Lower respiratory tract infections (including pneumonia) due to susceptible strains of bacteria 
  • Rhinosinusitis, acute bacterial due to susceptible strains of bacteria
  • Skin and skin structure infections due to susceptible strains of bacteria.

Amoxicillin may also be used off label to treat several different conditions such as anthrax and Lyme disease.

What is penicillin?

Penicillin was the first antibiotic discovered in 1928 by Dr. Alexander Fleming.

Penicillin V is an enhancement of the original penicillin discovered and was developed so that it could withstand stomach acid and be able to be taken orally.

Penicillin is considered a narrow-spectrum antibiotic because it is mainly effective against gram-positive aerobic organisms such as: 

  • Streptococcus pneumoniae
  • Groups A, B, C and G streptococci
  • Nonenterococcal group D streptococci 
  • Viridans group streptococci
  • Non-penicillinase producing staphylococcus.

Aminoglycosides may be combined with penicillin to improve its effect against group B streptococcus (S. agalactiae), S. viridans, and Enterococcus faecalis.

May also be used against susceptible gram-positive aerobic bacilli such as Bacillus anthracis, Corynebacterium diphtheriae, and Erysipelothrix rhusiopathiae.

Penicillins have limited activity against gram-negative organisms; however, they may be used in some cases to treat infections caused by Neisseria meningitides and Pasteurella.

Not generally used to treat anaerobic infections. Resistance patterns, susceptibility and treatment guidelines vary across regions.

Penicillin is approved to treat or prevent:

  • Dental infections involving gum tissue, such as fusospirochetosis (Vincent gingivitis and pharyngitis) in conjunction with dental care
  • Mild to moderately severe pneumococcal respiratory tract infections, including otitis media. 
  • Rheumatic fever and/or chorea prophylaxis 
  • Staphylococcal infections (penicillin G-sensitive) which may affect the skin or soft tissues
  • Mild-to-moderate Streptococcal infections (without bacteremia) of the upper respiratory tract, scarlet fever, and mild erysipelas. 

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