Brand Name: Albon, Primor

Available as 125 mg tablets, 250 mg tablets, 500 mg tablets, and oral suspension

Sulfadimethoxine is also the sulfa drug portion of the combination drug known as Primor (ormetoprim/sulfadimethoxine)

Background

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 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 working for Bayer, Gerhard Domagk. In 1939, he won the Nobel Prize for Medicine and Physiology 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 works by exploiting the bacterial cell's need for folic acid.  Folic acid, crucial for cell division, is made from para-aminobenzoic 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, 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.

How this Medication is Used

Sulfa drugs may have numerous uses and are most commonly used in synergistic combination products with antibiotics trimethoprim or ormetoprim. Sulfadimethoxine alone 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.

Interactions with other Drugs

Antacids may interfere with oral absorption.

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 a particularly good ability to penetrate into tissue that other antibiotics cannot.

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).

Sulfadimethoxine can promote the potential for kidney disease when combined with cyclosporine. Further, cyclosporine levels may be decreased, so it may not work as well as it would have alone.

Common Side Effects

The most common side effects are loss of appetite, vomiting, and diarrhea. These are usually minor and are generally helped by providing food with the medication.

The Doberman pinscher seems to be over-represented and complete recovery can be expected within one week of discontinuing the medication. The samoyed and miniature schnauzer breeds have also been considered over-represented.

Sulfa drugs of any kind can disrupt tear function. This typically 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 not taking the drug. If use is to continue for more than a week, tear production may require monitoring.

Doberman pinschers, samoyeds, and miniature schnauzers are considered sensitive to this medication.

Serious Side Effects

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 syndromes can occur in certain individuals. These syndromes mostly represent idiosyncratic reactions, which means their occurrence has nothing to do with the amount of drug given but instead is an unpredictable individual’s sensitivity to any dose.

Inability to Produce Adequate Tears (occurs in about 15% of dogs on sulfa medications)

Joint Inflammation
A broad inflammatory syndrome has been observed in some individuals sensitive to sulfas. 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.

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 when the offending drug is stopped. Any drug of any kind can produce a drug reaction in the skin; a sulfa drug is somewhat over-represented in cases of skin-related drug eruptions.

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

This medication can help cause urine-forming crystals or even stones. This is typically a problem with prolonged use or acidified urine.

Concerns and Cautions

• 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 dogs, 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 the inability to produce adequate tears (dry eye) is a possible side effect of sulfa drugs, it is prudent not to use them in patients who already have this condition independently. 
  
  
• This medication is not considered safe during pregnancy.
  
  
• As mentioned, the Doberman pinscher is predisposed to the immune-mediated side effects listed above. Sulfadimethoxine is best not used in this breed. The samoyed and miniature schnauzer have similar considerations.