Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066




(for veterinary information only)



125 mg, 250 mg, 500 mg



Sulfadimethoxine is an antibiotic of the sulfonamide class. We would be remiss not to mention the historical significance of this antibiotic class. The sulfa or “sulfonamide” class of antibiotics has earned a special place in history as the very first antibiotics ever developed and for the first time in human history domination over bacterial infection became readily feasible.

The first sulfa drug was synthesized in 1932 by a German scientist named Gerhard Domagk who was working for Bayer. He won the Nobel prize for Medicine and Physiology in 1939 for his work and numerous related sulfas were soon to follow. The sulfas saved numerous lives during World War II on both sides.

The sulfa class of antibiotics work by exploiting the bacterial cell's need for folic acid.  Folic acid, crucial for cell division, is made from para-amino benzoic acid (PABA) through a step by step process involving two enzymes. The sulfa drugs block the first enzyme rendering the cell unable to make folic acid. This is not a problem for pets on sulfa drugs because mammals, birds, and reptiles do not need to produce folic acid; they simply eat it in their diets in the form of green plants. If they do not eat it in their diets, they are further protected by having enzymes that are vastly less sensitive to sulfa blockade compared to bacterial enzymes. In short, the sulfa drug deprives bacteria of the folic acid they need without interfering with the folic acid available to the host.

Gerhard Domagk small wiki

Gerhard Domagk won
the Nobel Prize
in Medicine 1939
(Public Domain Image:
Wikimedia Commons)



Sulfa drugs may have numerous uses and are most commonly used in syngergistic combination products with the antibiotics: trimethoprim or ormetoprim. Sulfadimethoxine alone is is used almost exclusively for the treatment of intestinal parasites known as Coccidia. These parasites are single-celled organisms capable of causing intense diarrheas in their hosts.

Sulfadimethoxine is generally given once daily and can be given with or without food.



Antacids may interfere with oral absorption of this medication.

In combination with trimethoprim or ormetoprim, the sulfa antibiotics produce what is called a "sequential blockade" attacking folic acid synthesis in two locations of the enzyme sequence. This combination makes for an excellent broad spectrum antibacterial product with particularly excellent ability to penetrate into tissue that other antibiotics cannot penetrate. For more information click here.



Side effects of concern can be divided into two groups: those that are common and those that are potentially serious. Potentially serious side effects are generally rare (<4% incidence) and may be random (not related to how much medication is used).


The most common side effects are: appetite reduction, vomiting, diarrhea. These are usually minor and are generally mitigated by providing food with the medication.

Inability to produce adequate tears (occurs in about 15% of dogs on sulfa medications)

Sulfa drugs of any kind are capable of disrupting tear function. Classically, this occurs after long term therapy (i.e. weeks to months) of use but occasionally certain individuals suffer from dry eyes after only one dose of sulfa. In most cases, tear function resumes normally after the drug is discontinued but occasionally the effect is long term or permanent despite withdrawal of the drug.


Sulfa drugs have numerous potential side effects and though they may be rare, it is a good idea to become familiar with what to look for. The following are syndromes that can occur in certain individuals taking sulfa drugs. These syndromes mostly represent “idiosynchratic” reactions which mean their occurrence has nothing to do with the amount of drug given but instead represent an unpredictable individual’s sensitivity to any dose:

Joint inflammation

A broad inflammatory syndrome has been observed in some individuals sensitive to sulfas. This includes arthritis, fever, muscle soreness, kidney inflammation, and even inflammation in the eye. This syndrome has been formally studied and has been found to occur almost exclusively after a previous uneventful exposure to a sulfa drug and occurs 8-20 days after therapy has started. The Doberman pinscher seems to be over-represented and complete recovery can be expected within one week of discontinuing the medication.

No Doberman(Photocredit:

Skin rashes

Drug related skin reactions do not have characteristic appearances; in fact, they can have any appearance. They do, however, begin around the start of treatment with the offending drug and vanish with cessation of administration of the offending drug. Any drug of any kind can produce a drug reaction in the skin; Sulfa drugs are somewhat over-represented in cases of skin related drug eruptions.

Blood dyscrasias

Blood dyscrasias are abnormal blood cells or proportions of different blood cells. Blood dyscrasias might lead to immune dysfunction, bleeding tendency, or other problems depending on which blood cells are affected. With sulfas, loss of red blood cells, platelets, and white blood cells have been reported.  This syndrome is typically part of the joint inflammation syndrome.

This medication can precipitate in urine forming crystals or even stones. This is typically a problem with prolonged use or acidified urine.



  • It has been recommended that this medication not be used in patients with liver or kidney disease. In species other than dogs, sulfadimethoxine is removed from the body after it is “acetylated” in the liver and directly excreted by the liver. Patients with liver disease may not remove sulfadimethoxine properly from their bodies and may suffer toxic effects. In the dog, sulfadimethoxine is removed by the kidneys so the liver is not at risk.
  • Patients with known reactions against members of the sulfa class of antibiotics should not take this medication.
  • Since inability to produce adequate tears (“Dry Eye”) is a possible side effect of sulfa drugs, it is prudent not to use them in patients that already have this condition independently.
  • This medication is not considered safe in pregnancy.
  • As mentioned, the Doberman pinscher is predisposed to the immune-mediated side effects listed above. Sulfadimethoxine is best not used in this breed.


Short version (to help us comply with "Lizzie's Law")

Page last updated: 7/8/2020