Sulfadimethoxine/Ormetoprim
Sulfadimethoxine and Ormetoprim
Also known as: Primor · Rofenaid 40 · Romet 30 · NSC-95072 · ormetoprima · ormétoprime · ormetoprimum · Ro-5-9754 · solfadimetossina · solfadimetossipirimidina · sulphadimethoxine · Chemiosalfa · Deltin · Risulpir · Ritarsulfa · Sulfadren · Sulfastop · Sulfathox
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Susceptible infections (skin and soft tissue) | Initially 55 mg/kg (combined drug) PO on the first day of therapy, then 27.5 mg/kg PO once daily for at least 2 days after remission of clinical signs. Not approved for treatment longer than 21 days. | PO | q24h | Up to 21 days | 🌎 NA |
- Susceptible infections (skin and soft tissue): Dose is based on combined drug weight.
Birds
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Susceptible infections | Medicated premix: 113.5 grams sulfadimethoxine and 68.1 grams ormetoprim per pound in 50 lb bags | PO | Continuous in feed | Varies | 🌎 NA |
- Susceptible infections: FDA-approved for chickens (broilers, replacements), turkeys, ducks, & Chukar partridges. 5-day slaughter withdrawal. Do not feed to chickens over 16 weeks of age, or turkeys/ducks producing eggs for food.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Sulfadimethoxine/ormetoprim is a potentiated sulfonamide antimicrobial FDA-approved for the treatment of skin and soft tissue infections in dogs caused by susceptible strains of Staphylococcus aureus and E. coli.
Key Clinical Features:
- Convenience: Can be dosed once daily, unlike many other sulfa drugs.
- Tolerability: May have fewer adverse effects in dogs compared to trimethoprim/sulfadiazine.
- Limitations: It does not penetrate the prostate well, making it ineffective for bacterial prostatitis. It is also generally more expensive than generic trimethoprim/sulfa combinations.
Clinical Pearl: While resistance develops more slowly to the combination than to either drug alone, plasmid-mediated resistance is common in gram-negative organisms.
Mechanism of action
Sulfadimethoxine/ormetoprim is bactericidal due to the synergistic, sequential inhibition of enzymes in the bacterial folic acid synthesis pathway.
- Sulfadimethoxine (a sulfonamide) acts as a false substrate, competitively inhibiting the conversion of para-aminobenzoic acid (PABA) → dihydrofolic acid (DFA).
- Ormetoprim (a diaminopyrimidine) blocks the subsequent conversion of DFA → tetrahydrofolic acid (THFA) by reversibly inhibiting the enzyme dihydrofolate reductase.
This dual blockade halts bacterial thymidine synthesis, ultimately preventing DNA replication. It has a broad spectrum of activity against many Gram-positive (e.g., Staphylococcus, Streptococcus, Nocardia) and Gram-negative organisms (excluding Pseudomonas aeruginosa), as well as certain protozoa (Pneumocystis, Coccidia, Toxoplasma).
Safety & warnings
Contraindications
- Hypersensitivity to sulfonamides, thiazides, or sulfonylurea agents
- Severe renal or hepatic impairment
- Marked liver parenchymal damage
- Blood dyscrasias
- Doberman Pinschers (highly susceptible to sulfonamide-induced poly-systemic immune complex disease)
Adverse effects
- Keratoconjunctivitis sicca (KCS/dry eye) - potentially irreversible
- Acute neutrophilic hepatitis with icterus
- Vomiting
- Anorexia
- Diarrhea
- Fever
- Hemolytic anemia
- Urticaria
- Polyarthritis
- Facial swelling
- Polydipsia and polyuria
- Cholestasis
- Hypersensitivity reactions (Type I anaphylaxis or Type III serum sickness)
- Hypothyroidism (with extended use)
- Crystalluria and hematuria (especially in cats)
- Leukopenias and anemias
Precautions
Use with caution in patients with pre-existing hepatic or thyroid disease, diminished renal function, urinary obstruction, or urolithiasis. Hypersensitivity reactions appear more common in large breed dogs. Potentially teratogenic (cleft palate reported in lab animals with TMP/sulfa); weigh risk versus benefit in pregnant animals.
Drug interactions
Specific interactions are not explicitly noted for this combination, but it is expected to share the interaction profile of trimethoprim/sulfa (e.g., potential antagonism with cyclosporine, increased risk of bleeding with anticoagulants).
Monitoring
- Clinical efficacy and resolution of infection
- Adverse effects (especially signs of hypersensitivity or liver issues)
- Tear production via Schirmer Tear Test in dogs (baseline, at 5 days post-initiation, and then every 2-3 weeks)
Pharmacokinetics
Distribution
Ormetoprim does not attain clinically effective levels in the prostate for treating bacterial prostatic infections.
Overdose
In experimental dog studies, doses >80 mg/kg resulted in slight tremors and increased motor activity. Higher doses may cause depression, anorexia, or seizures.
Treatment: For very high oral overdoses, empty the gut using standard precautions and protocols. Treat clinical signs supportively and symptomatically.
Available products
Formulations
- Tablets
- Medicated premix
Veterinary
- Sulfadimethoxine/Ormetoprim Tablets (scored): 120 mg (100 mg sulfa/20 mg ormetoprim), 240 mg, 600 mg, 1200 mg (Primor)
- Sulfadimethoxine/Ormetoprim medicated premix (Rofenaid 40, Romet 30)
Regulatory status
No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Unless otherwise instructed by the manufacturer, store tablets in tight, light-resistant containers at room temperature.
Client information
- Hydration is critical: Ensure your pet has free access to fresh water at all times to prevent dehydration and the formation of urinary crystals.
- Watch the eyes: Monitor for signs of "dry eye" (squinting, redness, thick mucoid discharge). Contact your veterinarian immediately if these occur, as this can cause permanent eye damage.
- Allergic reactions: Stop the medication and contact your vet if you notice facial swelling, hives, vomiting, fever, joint pain, or severe lethargy.
- Give the medication exactly as prescribed, and do not skip doses. Even if your pet looks better, finish the entire course to prevent bacterial resistance.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.