(for veterinary information only)

50 mg, 100 mg, 125 mg, 150 mg, 200 mg,
250 mg, 400 mg, 500 mg, 875 mg    


Alexander Fleming wiki

Howard Walter Florey 1945 wiki

Ernst Boris Chain 1945 wiki

Alexander Fleming

 Howard Florey

Ernst Chain

(Photocredit all three images: Wikimedia Commons)

Thanks to work by Alexander Fleming (1881-1955), Howard Florey ( 1898-1968) and Ernst Chain (1906-1979), penicillin was first produced on a large scale for human use in 1943. At that time, the development of a pill that could reliably kill bacteria was remarkable and many lives were saved during World War II because this medication was available.

But quickly, it became obvious that this new "wonder drug" could bear improvement. For example:

  • Penicillin is not well absorbed from the intestinal tract meaning that at least 70% of an oral dose is wasted.
  • Penicillin is also a short-acting medication, with half of the amount circulating being removed from the body every half hour.
  • Not all bacteria have the type of cell wall which is susceptible to destruction by penicillin. (Bacteria are classified as Gram negative or Gram positive, depending on the cell wall characteristics. Penicillin is able to punch holes through the Gram positive cell wall but is not very effective against the Gram negative cell wall.)
  • Staphylococci (an important group of bacteria) have developed an enzyme to break the penicillin molecule apart and are thus rarely susceptible to penicillin.

Amoxicillin represents a synthetic improvement upon the original penicillin molecule. Amoxicillin is better able to resist damage from stomach acid so less of an oral dose is wasted. While it is still susceptible to destruction by staphylococcal enzymes, it does have a much broader spectrum against the Gram negative cell wall and is able to last a bit longer.


Amoxicillin is regarded as having a fairly broad spectrum against many bacteria thus it is used both on organisms known to be sensitive to it plus it is a good selection when the sensitivity of bacteria is unknown. It is especially helpful in anaerobic infections (those which grow without the benefit of oxygen). Typical uses might include:

It should be noted that staphylococcal infections are NOT sensitive to this medication with two exceptions:

  • Staph infections in the bladder are sensitive to amoxicillin simply because the kidney concentrates such a large amount of amoxicillin in the bladder. The Staph enzymes are overwhelmed by the huge concentration of antibiotic and the Staph organism is killed.
  • Staph infections are sensitive to amoxicillin if clavulanic acid is given concurrently to protect amoxicillin from the Staph enzymes. (Amoxicillin/Clavulanic acid combinations are marketed under the names Clavamox and Augmentin.)

In these two situations, amoxicillin should prevail over a Staph infection; however, recent times have created yet another special situation: Methicillin-Resistant Staphylococci. These bacteria have mutated, have less predictable sensitivity, and do not follow the above rules for Staph sensitivity.

Methicillin-Resisant Staphylococcus aureus (also called "MRSA") is a human bacterium which can be transmitted to animals which can in turn re-transfer the bacteria to back to humans. Methicillin-Resistant Staphylococcus pseudointermedius (often called "MRSI") is a bacterium of pet species. These mutant forms of Staph make antibiotic selection more difficult and culture/sensitivity testing may be needed to select an effective anti-Staph antibiotic.


When the organism in a serious infection cannot be isolated, a common strategy is to attempt to "cover" for all possible bacteria. Amoxicillin is frequently used in combination with other antibiotics for this purpose.

Clavulanic acid may be added to amoxicillin to increase amoxicillin's spectrum against staphylococcal bacteria.

Amoxicillin is believed to synergize with members of the fluorquinolone class of antibiotics (enrofloxacin, orbifloxacin etc.)


Some individuals experience nausea with this medication. Giving the medication with food seems to reduce this effect.


The oral suspension should be refrigerated, though if it is mistakenly left out of the refrigerator for one day, this is not a problem. The oral suspension should be discarded after 2 weeks.

Amoxicillin may be given with or without food.

Amoxicillin will cross the placenta in a pregnant patient but is felt to be safe for use during pregnancy.

Page last updated: 2/9/2013