Last updated: July 31, 2026
Sulfisoxazole is an older sulfonamide antibacterial with limited contemporary use. No meaningful late-stage clinical-development program, active branded market, or significant patent barrier supports near-term commercial expansion. The product’s remaining value is concentrated in low-cost generic supply, selected topical or ophthalmic applications, and combination products. Current revenue is likely small relative to modern antibacterial agents, and market growth is expected to remain flat or decline unless a new formulation or specialized indication is developed.
What is sulfisoxazole and how is it used?
Sulfisoxazole is a synthetic sulfonamide that inhibits bacterial dihydropteroate synthase and disrupts folate synthesis. It is bacteriostatic rather than bactericidal and has activity against selected susceptible gram-positive and gram-negative organisms.
Historically, sulfisoxazole was marketed under the Gantrisin brand and used for urinary tract infections, respiratory infections, otitis media, and other susceptible bacterial infections. It was also used in combination with erythromycin for pediatric otitis media and in ophthalmic products for superficial ocular infections.
Its clinical relevance declined because of:
- Widespread bacterial resistance to sulfonamides.
- More effective oral antibiotics with broader and more predictable coverage.
- Safety concerns, including hypersensitivity, crystalluria, blood dyscrasias, and severe cutaneous reactions.
- Reduced use of systemic sulfonamides in children and other vulnerable populations.
- Withdrawal or discontinuation of several legacy branded and combination products.
Current product availability varies by country and dosage form. Historical United States labeling identifies sulfisoxazole as an antibacterial for susceptible infections, but active commercial availability is substantially narrower than in the product’s peak market period (U.S. Food and Drug Administration [FDA], 2024a).
What is the current clinical-trial status of sulfisoxazole?
Sulfisoxazole does not have a visible modern clinical-development pipeline comparable with newer antibiotics. ClinicalTrials.gov searches historically identify few or no active pivotal studies focused on sulfisoxazole as a standalone investigational product. The available evidence base is largely composed of older controlled trials, observational studies, pharmacology studies, and historical pediatric infection research.
Clinical development status
| Development category |
Current assessment |
| Phase 1 studies |
No meaningful contemporary program |
| Phase 2 studies |
No active product-development signal |
| Phase 3 studies |
No current pivotal program identified |
| Registration-enabling studies |
None apparent for a new sulfisoxazole product |
| Combination studies |
Historical and limited |
| Pediatric use |
Supported mainly by legacy labeling and older literature |
| Ophthalmic use |
Limited commercial and clinical relevance |
| Resistance research |
Primarily historical or class-based rather than drug-development focused |
The absence of a current trial program reflects commercial and clinical obsolescence rather than an unresolved regulatory pathway. A sponsor seeking approval for a new sulfisoxazole formulation would likely need to generate contemporary chemistry, manufacturing and controls data, pharmacokinetic evidence, safety data, and efficacy evidence for the proposed indication.
What FDA regulatory status does sulfisoxazole have?
Sulfisoxazole has a long history of FDA-regulated use, but the status of individual products depends on the manufacturer, dosage form, and application.
The former Gantrisin products were associated with legacy FDA approvals and labeling. Historical formulations included oral tablets, oral liquids, combination products, and ophthalmic preparations. Some of these products have been discontinued or are no longer actively marketed.
The FDA’s current databases should be used to distinguish:
- Approved products that remain marketed.
- Approved products that have been discontinued.
- Products withdrawn for commercial reasons.
- Products withdrawn for safety or effectiveness reasons.
- Legacy applications with no current commercial supply.
A discontinued product is not automatically a failed product under FDA law. The FDA may classify a product as discontinued for commercial reasons while leaving the underlying approval record intact. This distinction matters when evaluating abbreviated new drug application opportunities and whether an applicant can rely on a listed reference product (FDA, 2024b).
What is the Orange Book status of sulfisoxazole?
Sulfisoxazole’s Orange Book position is limited compared with current high-value antibiotics. The relevant analysis must be performed at the specific product and dosage-form level because historical approvals may not correspond to products currently sold.
For an active generic opportunity, the key questions are:
- Whether an approved reference-listed drug remains identifiable.
- Whether the reference product is currently marketed.
- Whether the drug has active listed patents or exclusivity.
- Whether a proposed generic can qualify for an ANDA pathway.
- Whether the dosage form has a suitable reference product.
- Whether the product is subject to discontinued-drug or withdrawn-approval complications.
No commercially important active patent estate is generally associated with the original sulfisoxazole molecule. Any remaining regulatory protection would more likely involve a specific formulation, combination, manufacturing process, or later-developed delivery system than the basic active ingredient.
The Orange Book should be reviewed for current patent listings and exclusivity codes before filing or licensing decisions. Historical products may have no active patent listings even when the FDA approval record remains accessible (FDA, 2024c).
What patents protect sulfisoxazole?
The core sulfisoxazole compound is an old small molecule and is outside the practical life of original composition-of-matter patent protection. Any foundational patents would have expired decades ago.
Patent categories
| Patent category |
Commercial relevance |
| Original compound patent |
Expired |
| Basic antibacterial use patent |
Expired or commercially irrelevant |
| Legacy oral formulation patent |
Expired |
| Historical combination-product patent |
Generally expired |
| New ophthalmic delivery system |
Potentially relevant if recently developed |
| Sustained-release formulation |
Potentially patentable but requires a new product |
| Manufacturing process patent |
Possible, but usually narrow |
| Salt, polymorph, or excipient patent |
Limited value unless linked to a differentiated product |
A modern sulfisoxazole patent strategy would need to focus on a technically differentiated product. Potential targets could include a preservative-free ophthalmic formulation, improved solubility, sustained delivery, reduced crystalluria, pediatric taste masking, or a combination product with a defined clinical advantage. A patent on a routine dosage adjustment or conventional excipient combination would face substantial validity and obviousness risk.
When does sulfisoxazole lose exclusivity?
Sulfisoxazole lost meaningful product exclusivity long ago. There is no current blockbuster-style exclusivity cliff comparable with a protected branded antibiotic.
The relevant timeline is:
| Event |
Timing |
| Original sulfonamide development |
20th century |
| Gantrisin-era commercial use |
Historical |
| Core compound exclusivity |
Expired decades ago |
| Generic competition |
Established for many years |
| Branded market decline |
Historical |
| Current commercial position |
Legacy generic or limited-market product |
Any new sulfisoxazole product would receive exclusivity only through a new regulatory and patent strategy. Depending on the product, possible protections could include five-year new chemical entity exclusivity, three-year exclusivity for certain supplemental clinical investigations, orphan-drug exclusivity if the indication qualified, or pediatric exclusivity. These protections would apply to the new approved product and indication, not revive exclusivity for the old molecule generally.
What formulation patents could protect a new sulfisoxazole product?
Formulation development is the main potential route to commercial differentiation.
Ophthalmic formulations
An ophthalmic product could target:
- Improved corneal residence time.
- Reduced irritation.
- Lower preservative exposure.
- Better stability.
- Unit-dose packaging.
- Enhanced penetration into ocular tissues.
The commercial opportunity would depend on evidence of clinical or usability advantages over established generic ophthalmic antibacterials.
Pediatric oral formulations
A pediatric product could use:
- Taste masking.
- Improved suspension stability.
- Reduced dosing frequency.
- Unit-dose packaging.
- Lower excipient burden.
- Compatibility with pediatric administration devices.
These features could support formulation patents, but market protection would remain narrow unless the product obtained a differentiated FDA-approved indication.
Combination products
A sulfisoxazole combination with another antibacterial or anti-inflammatory agent could obtain protection for a specific formulation or method of use. Combination patents would face prior-art scrutiny because sulfonamide combinations have been used for decades.
Which companies are challenging sulfisoxazole patents?
No major current Paragraph IV campaign or high-profile sulfisoxazole patent dispute is evident in the public record. The molecule is too old and commercially limited to support the type of branded-generic litigation seen with protected oncology, immunology, or specialty drugs.
Paragraph IV risk
A new ANDA sponsor would generally evaluate:
- Paragraph I certification if no relevant patents are listed.
- Paragraph II certification if listed patents have expired.
- Paragraph III certification for future expiration.
- Paragraph IV certification if listed patents are alleged invalid or not infringed.
- Section viii carve-outs for protected methods of use.
For legacy sulfisoxazole products, Paragraph IV litigation risk is likely low. The more significant barriers would be reference-product identification, product availability, bioequivalence, stability, and commercial scale.
What patent litigation and settlement agreements affect sulfisoxazole?
No material current patent litigation or settlement agreement is associated with sulfisoxazole as a major commercial asset. Historical litigation, if any, would have limited impact on present market entry because core patent rights have expired.
A future dispute would more likely involve:
- A newly patented ophthalmic delivery system.
- A pediatric formulation.
- A combination product.
- A manufacturing process.
- A branded product alleging infringement by a generic formulation.
Such litigation would be product-specific and would not restore broad exclusivity for sulfisoxazole.
What is the sulfisoxazole market size and commercial outlook?
Public pharmaceutical datasets generally do not report sulfisoxazole as a large standalone market. Sales are fragmented by country, dosage form, manufacturer, and combination product. The United States market appears to be limited compared with current oral and ophthalmic antibiotics.
Market drivers
Positive factors include:
- Low active pharmaceutical ingredient cost.
- Existing manufacturing knowledge.
- Potential use in price-sensitive markets.
- Possible niche demand for ophthalmic or pediatric formulations.
- Low probability of patent-driven competition.
Negative factors include:
- Resistance concerns.
- Generic price compression.
- Limited physician familiarity.
- Substitution by newer antibiotics.
- Weak reimbursement economics.
- Potential supply discontinuity.
- Small addressable patient population in developed markets.
Market projection
| Period |
Expected market direction |
Main rationale |
| 2025-2027 |
Flat to declining |
Mature generic market and limited clinical adoption |
| 2028-2030 |
Flat in low-income markets; declining in developed markets |
Continued substitution and resistance |
| Beyond 2030 |
Niche-only outlook |
Requires a differentiated formulation or new indication |
A numerical global revenue forecast would have low reliability because sulfisoxazole is not consistently separated from broader sulfonamide or generic-antibiotic datasets. The defensible projection is qualitative: stable or declining demand, with limited upside from specialty formulations.
How does sulfisoxazole compare with competing antibiotics?
| Attribute |
Sulfisoxazole |
Trimethoprim-sulfamethoxazole |
Modern ophthalmic antibiotics |
| Patent position |
Expired core rights |
Mostly generic, formulation-specific opportunities |
Mixed, depending on product |
| Clinical use |
Limited and historical |
Broad continuing use |
Active use in ocular infections |
| Resistance concern |
High |
Significant but clinically established |
Product- and pathogen-specific |
| Market scale |
Small |
Material generic market |
Larger in selected markets |
| Trial activity |
Minimal |
Ongoing clinical research |
Ongoing product and indication development |
| Commercial potential |
Low |
Moderate |
Moderate to high for differentiated products |
Trimethoprim-sulfamethoxazole has retained broader utility because the combination provides complementary mechanisms and remains established in several bacterial indications. Fluoroquinolone and aminoglycoside ophthalmic products also have stronger contemporary market positions.
What generic entry risks exist for sulfisoxazole?
Generic entry risk is high from an intellectual-property standpoint and low from a market-attractiveness standpoint. A sponsor could face limited patent opposition but still reject the opportunity because of:
- Small market size.
- Low selling prices.
- Limited active distributors.
- Procurement-driven margin pressure.
- API sourcing constraints.
- Uncertain reference-product status.
- Weak physician demand.
- Potential regulatory requirements for discontinued or legacy products.
The most viable entry model would be a low-cost, reliable generic supplied through institutional, government, or selected international channels. A premium launch would require clinical or formulation differentiation.
Key Takeaways
- Sulfisoxazole is a legacy sulfonamide antibacterial with limited current clinical use.
- No meaningful contemporary pivotal-trial pipeline is apparent.
- Core compound patent protection expired decades ago.
- Current Orange Book value depends on the specific dosage form and reference product.
- Paragraph IV and patent-litigation risk are likely low.
- Regulatory and commercial execution risks exceed intellectual-property risks.
- Market demand is expected to remain flat or decline through 2030.
- The strongest potential opportunity is a differentiated ophthalmic, pediatric, or controlled-delivery formulation.
- A conventional generic sulfisoxazole launch would likely have low revenue potential and significant price pressure.
- No credible blockbuster or high-growth commercial scenario exists without a new indication or delivery technology.
Frequently Asked Questions
Is sulfisoxazole still available in the United States?
Availability is limited and depends on the specific formulation and manufacturer. Historical branded products such as Gantrisin are no longer the primary commercial reference for the drug.
Is sulfisoxazole the same as sulfamethoxazole?
No. Both are sulfonamide antibacterials, but sulfisoxazole and sulfamethoxazole are different active ingredients. Sulfamethoxazole is more commonly used today, particularly in combination with trimethoprim.
Can a company obtain new patents on sulfisoxazole?
A company cannot obtain a new composition patent on the old molecule merely by claiming the existing compound. It may pursue patents covering a novel formulation, delivery system, manufacturing process, or specific method of use.
Does sulfisoxazole have biosimilar competition?
No. Sulfisoxazole is a chemically synthesized small-molecule drug, not a biologic. The relevant competitors are generic products, not biosimilars.
Would sulfisoxazole qualify for a 505(b)(2) application?
A 505(b)(2) pathway could be relevant to a novel formulation, route of administration, or combination product that relies partly on existing sulfisoxazole data. Eligibility would depend on the proposed product and the FDA’s reference to prior findings.
References
- U.S. Food and Drug Administration. (2024a). Sulfisoxazole drug labeling and historical product information. https://www.accessdata.fda.gov
- U.S. Food and Drug Administration. (2024b). Drugs@FDA: FDA-approved drugs database. https://www.accessdata.fda.gov/scripts/cder/daf/
- U.S. Food and Drug Administration. (2024c). Approved drug products with therapeutic equivalence evaluations: Orange Book. https://www.fda.gov/drugs/drug-approvals-and-databases/approved-drug-products-therapeutic-equivalence-evaluations-orange-book
- ClinicalTrials.gov. (2025). Search results for sulfisoxazole clinical studies. U.S. National Library of Medicine. https://clinicaltrials.gov/
- DailyMed. (2025). Sulfisoxazole drug labeling database. National Library of Medicine. https://dailymed.nlm.nih.gov/
