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Amoxicillin (Numerous Brand Names)
Revised: November 22, 2022
Published: January 01, 2001

(For veterinary information only)

The size of the tablet/medication is NOT an indication of a proper dose. Never administer any drug without your veterinarian's input. Serious side effects or death can occur if you use drugs on your pet without your veterinarian's advice. 

It is our policy not to give dosing information over the internet.

Available in oral suspension; 250 mg and 500 mg capsules; and 40 mg, 50 mg, 100 mg, 125mg, 150 mg, 200 mg, 250 mg, 400 mg, 500 mg, 875 mg tablets


Penicillin was first produced on a large scale for human use in 1943 through the work of Alexander Fleming (1881-1955), Howard Florey (1898-1968), and Ernst Chain (1906-1979. At the 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 it quickly became apparent that this new wonder drug could bear improvement. For example: 

  • Penicillin is not well absorbed from the intestinal tract, meaning that at least 70 percent of an oral dose is wasted.

  • Penicillin is a short-acting medication, and half of the amount circulating is removed from the body every half hour.
  • Not all bacteria have the type of cell wall that is susceptible to destruction by penicillin. (Bacteria are classified as gram-negative or gram-positive, depending on the cell wall characteristics. Penicillin can punch holes through the gram-positive cell wall but is not very effective against the gram-negative cell wall.)
  • Staphylococci (a significant 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 has a much broader spectrum against the gram-negative cell wall and can last a bit longer.

How this Medication is Used

Amoxicillin is regarded as having a fairly broad spectrum against many bacteria so it is used both on organisms known to be sensitive to it, and 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:

Staphylococcal Infections

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

  • Staph infections in the bladder are frequently sensitive to amoxicillin simply because the kidney concentrates such a large amount of amoxicillin in the urine. The Staph protective enzymes are overwhelmed by the huge concentration of antibiotics, 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 Staphylococcus aureus (also called MRSA) is a human bacterium that can be transmitted to animals, which can, in turn, re-transfer the bacteria back to humans. Methicillin-resistant Staphylococcus pseudointermedius (often called MRSP) 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.

A few words regarding Methicillin-resistant Staphylococci as these are in the news periodically: These bacteria have mutated, have less predictable sensitivity, and do not follow the above rules for Staph sensitivity. In the event of a Methicillin-resistant Staph infection, a culture is needed to determine the sensitivity of the organism, even if the infection is in the bladder where high levels of amoxicillin are present, as described.

Interactions with Other Drugs

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 fluoroquinolone class of antibiotics (enrofloxacin, orbifloxacin, etc.)

Side Effects

Some individuals experience nausea, diarrhea, or appetite loss. These are generally mild, and giving the medication with food should reduce it.

Serious side effects are unusual, but the following has been reported: fever, rashes, and bone marrow suppression.

Concerns and Cautions

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

If a dose is accidentally skipped, give the next dose when it is remembered and time the subsequent dose accordingly.

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.

Amoxicillin is a time-dependent antibiotic, which means it must be given at specific intervals to maintain a therapeutic level. Amoxicillin is given two to three times daily, depending on the type of infection.

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