Sulfadimethoxine

BRAND NAMES

Some of the most well-known brand names for the veterinary antimicrobial sulfadimethoxine include Albon, Di-Methox, and Primor. 

• Albon: Available as an oral suspension and tablets, this is a commonly used brand for treating infections, especially coccidiosis, in dogs and cats.

• Di-Methox: Another brand name for sulfadimethoxine. 

MECHANISM OF ACTION

Sulfadimethoxine works by interfering with bacterial folic acid synthesis, which is essential for the production of nucleotides and, ultimately, DNA and RNA. Specifically, it acts as a competitive inhibitor of the enzyme dihydropteroate synthase. This enzyme normally catalyzes the incorporation of para-aminobenzoic acid (PABA) into dihydropteroate, a key step in the folic acid pathway. By mimicking PABA, sulfadimethoxine blocks this step, preventing the bacteria from producing folic acid. Since bacteria cannot take up folic acid from their environment and must synthesize it themselves, this inhibition disrupts their growth and replication. Because mammalian cells obtain folic acid from their diet and do not synthesize it, sulfadimethoxine selectively targets bacteria without directly harming the host’s cells.

PHARMACOKINETICS

Absorption

Sulfadimethoxine is typically well absorbed from the gastrointestinal tract in most animal species, although the rate and extent of absorption may differ depending on the species, formulation, and route of administration. Its long-acting nature is attributed to its high degree of plasma protein binding and effective renal reabsorption, which help maintain therapeutic levels in the body for an extended period.

Distribution

Sulfadimethoxine has a moderate volume of distribution, typically ranging from 0.2 to 0.6 L/kg, indicating good distribution in body fluids but limited tissue penetration. Its high plasma protein binding contributes to its long duration of action.

Metabolism

Sulfadimethoxine is metabolized primarily in the liver, where it undergoes acetylation and glucuronidation to form inactive metabolites. The extent of metabolism can vary among species, but a significant portion of the drug is usually excreted unchanged. These metabolic processes aid in detoxification and facilitate elimination through the kidneys, contributing to the drug’s long half-life and sustained therapeutic effect.

Excretion

Sulfadimethoxine is excreted mainly through the kidneys, primarily via glomerular filtration and tubular reabsorption. A portion of the drug is eliminated unchanged, while the rest is excreted as inactive metabolites. Its efficient renal reabsorption contributes to its long half-life and prolonged therapeutic activity. Small amounts may also be excreted in bile or feces, depending on the species.

PHARMACODYNAMICS

Sulfadimethoxine is a bacteriostatic antibiotic that works by inhibiting dihydropteroate synthase, blocking folic acid synthesis needed for bacterial growth and replication. Its effect is time-dependent, requiring sustained drug levels for optimal activity.

ADMINISTRATION

Sulfadimethoxine can be administered orally or by injection, depending on the species and condition being treated. Oral formulations (tablets, powders, or liquids) are commonly used for ease of dosing, while intramuscular or intravenous routes may be used in severe infections. It is often given as a loading dose followed by lower maintenance doses to maintain effective blood levels due to its long half-life.

DOSAGE AND STRENGTH

The dosage and strength of sulfadimethoxine vary depending on the species, type of infection, and formulation used. Generally, treatment begins with a loading dose of 50 mg/kg body weight on the first day, followed by a maintenance dose of 25 mg/kg once daily. In veterinary formulations, sulfadimethoxine is commonly available in oral tablets, oral suspensions, and injectable forms with strengths such as 125 mg, 250 mg, 500 mg tablets, or 50 mg/mL injectable solutions. Dosage should always be adjusted according to the animal’s condition and veterinary guidance.

FOOD INTERACTIONS

Sulfadimethoxine has minimal food interactions, and it can generally be given with or without food. However, administering it with food may help reduce potential gastrointestinal irritation in some animals. High-protein diets or feeds are not known to significantly affect its absorption or efficacy. Adequate water intake should be ensured during treatment to help prevent crystal formation in the urine and support proper drug excretion.

DRUG INTERACTIONS

Sulfadimethoxine can interact with various drugs, potentially affecting their safety and effectiveness. When combined with other sulfonamides or diuretics such as thiazides, there is an increased risk of kidney damage and crystalluria due to reduced solubility in the urine. It can also enhance the effects of anticoagulants like warfarin, raising the risk of bleeding. Additionally, sulfadimethoxine may increase the toxicity of drugs like phenytoin and methotrexate by interfering with their metabolism or protein binding. Urine acidifying agents can further reduce sulfadimethoxine’s solubility, increasing the chance of crystal formation. Therefore, careful monitoring is essential when sulfadimethoxine is used alongside other medications.

CONTRAINDICATIONS

Sulfadimethoxine is contraindicated in animals with a known allergy to sulfonamides due to the risk of severe hypersensitivity reactions. It should be avoided in animals with liver or kidney impairment, as these conditions can affect the drug’s metabolism and excretion, increasing the risk of toxicity. Use is also contraindicated in animals with blood disorders like anemia or leukopenia, since sulfonamides can sometimes cause bone marrow suppression. Additionally, it should not be used in pregnant or lactating animals unless clearly indicated, as it may cross the placenta or be secreted in milk, potentially harming the offspring.

SIDE EFFECTS

• Gastrointestinal upset.

• Increased drinking and urination.

• Lethargy.

• Keratoconjunctivitis sicca (dry eye).

• Allergic or immune-mediated reactions.

• Blood disorders (dyscrasias).

• Crystalluria.

• Liver or kidney damage.

• Skin rashes and hives (urticaria).

• Facial swelling.

• Joint inflammation (polyarthritis).

OVERDOSE

• Loss of appetite (anorexia).

• Vomiting and diarrhea.

• Dehydration.

• Crystalluria (crystals in urine).

• Increased thirst and urination.

• Lethargy or weakness.

• Jaundice (from liver involvement).

• Anemia or other blood abnormalities.

• Incoordination or other neurological signs.

• Possible kidney or liver damage in severe cases.

TOXICITY

Sulfadimethoxine toxicity occurs when excessive doses or prolonged use lead to harmful effects, especially in animals with impaired kidney or liver function. Toxicity is primarily associated with crystal formation in the kidneys (crystalluria), which can cause renal damage and reduced urine output. Other signs include vomiting, diarrhea, loss of appetite, dehydration, jaundice, and in severe cases, blood disorders such as anemia or leukopenia due to bone marrow suppression. Chronic exposure may also lead to liver dysfunction or neurological effects like lethargy and incoordination. Ensuring adequate hydration and avoiding overdosing are key preventive measures against sulfadimethoxine toxicity.