Sulfasalazine

Sulfasalazine	Last updated: 12/09/2025
(Also known as: salicylazosulfapyridine; sulphasalazine)

GENERAL INFORMATION

Description

A widely used human and veterinary sulfonamide antibacterial and immunosuppressive medication

Examples of veterinary uses

Used to treat rheumatoid arthritis, inflammatory bowl disease, colitis & Crohn's disease

Examples of species treated

Dogs; Cats

Approval status

VMR 2013/2033 approval status (GB/UK)

Not approved

EU Regulatory approval status

Not approved

Chemical structure

Isomerism

Sulfasalazine exhibits structural isomerism through its unique molecular architecture, but it does not possess stereoisomers or geometric isomers. The molecule is composed of two distinct pharmacophores: sulfapyridine and mesalazine, linked via an azo bond (-N=N-). This azo linkage creates a rigid connection between the two aromatic rings, but it does not result in cis-trans isomerism. However, sulfasalazine can undergo tautomerism and ionization-based isomerism in solution, particularly under physiological conditions. The sulfonamide and carboxylic acid groups can exist in different protonation states, influencing the molecule’s conformation and interaction with biological targets. Additionally, once ingested, colonic bacteria cleave the azo bond, releasing the two active components, each of which can undergo acetylation, further diversifying their metabolic forms.

Chemical formula

C₁₈H₁₄N₄O₅S

Canonical SMILES

C1=CC=NC(=C1)NS(=O)(=O)C2=CC=C(C=C2)NN=C3C=CC(=O)C(=C3)C(=O)O

Isomeric SMILES

No data

International Chemical Identifier key (InChIKey)

OQANPHBRHBJGNZ-UHFFFAOYSA-N

International Chemical Identifier (InChI)

InChI=1S/C18H14N4O5S/c23-16-9-6-13(11-15(16)18(24)25)21-20-12-4-7-14(8-5-12)28(26,27)22-17-3-1-2-10-19-17/h1-11,20H,(H,19,22)(H,24,25)

2D structure diagram/image available?

Yes

Cambridge Crystallographic Data Centre diagrams

Common Name	Relationship	Link
sulfasalazine	-

General status

Veterinary substance type

Medicinal drug: anti-bacterial; anti-inflammatory

Substance groups

Aminosalicylate

Minimum active substance purity

Known relevant impurities

Substance origin

Synthetic

Mode of action

Works by reducing the production of chemicals that mediate inflammation such as inflammatory cytokines and eicosanoids.

Molecular targets

[Arachidonate 5-lipoxygenase, Inhibitor], [Prostaglandin G/H synthase 2, Inhibitor], [Prostaglandin G/H synthase 1, Inhibitor], [Peroxisome proliferator-activated receptor gamma, Agonist], [Inhibitor of nuclear factor kappa-B kinase subunit alpha, Inhibitor], [Inhibitor of nuclear factor kappa-B kinase subunit beta, Inhibitor], [Cystine/glutamate transporter, Inhibitor]

CAS RN

599-79-1

EC number

209-974-3

CIPAC number

US EPA chemical code

PubChem CID

Therapeutic Class

Intestinal antiinflammatory agents: Aminosalicylic acid and similar agents

ATCvet Code

QA07EC01

Controlled Drug?

Regulation 37/2010 MRL Classification

Molecular mass

398.39

PIN (Preferred Identification Name)

2-hydroxy-5-((E)-2-(4-((pyridin-2-yl)sulfamoyl)phenyl)diazen-1-yl)benzoic acid

IUPAC name

2-hydroxy-5-((E)-2-(4-((pyridin-2-yl)sulfamoyl)phenyl)diazen-1-yl)benzoic acid

CAS name

5-((para-(2-pyridylsulfamoyl)phenyl)azo)-salicylic acid

Forever chemical

Other status information

Relevant Environmental Water Quality Standards

Physical state

Brown to orange coloured powder

Commercial

Property

Value

Availability status

Current

Introduction & key dates

1938, first synthesised; Early 1940s, introduced; 1950s, first approved USA

Example manufacturers & suppliers of products using this active now or historically

Pharmacia
Active Pharma Supplies Ltd

Example products using this active

Salazopyrin

Formulation and application details

Usually supplied as tablets for oral administration. Human brands often adapted for animal use, off label.

Commercial production

Sulfasalazine is synthesised through a two-step chemical process involving diazotisation and azo coupling. The production begins with sulfapyridine, which undergoes diazotisation by reacting with sodium nitrite in an acidic aqueous solution, typically using hydrochloric acid, to form a diazonium salt. This reactive intermediate is then coupled with salicylic acid in an alkaline medium, usually a sodium hydroxide solution, to form the azo-linked sulfasalazine molecule. The resulting crude product is then purified, often through crystallisation or filtration, to remove impurities and achieve pharmaceutical-grade quality.

Impact on climate of production and use

Published GHG data is not available for most pharmaceuticals. However, according to industry, global averages suggest producing 1 kg of a typical active pharmaceutical ingredient can range from 10 to 100 kg CO₂e for small molecule drugs and potentially up to 1000 kg CO₂e for complex biologicals such as vaccines, depending on the drug type, its formulation, complexity of synthesis, solvent recovery, and energy sources used.

ENVIRONMENTAL FATE

Property

Value

Source; quality score; and other information

Interpretation

Solubility - In water at 20 °C (mg l⁻¹)

Solubility - In organic solvents at 20 °C (mg l⁻¹)

Melting point (°C)

Decomposes on melting

Boiling point (°C)

Degradation point (°C)

220

Flashpoint (°C)

Octanol-water partition coefficient at pH 7, 20 °C

3.16 X 10⁰²

Calculated

Log P

2.5

Low

Fat solubility of residues

Solubility

Data type

Density (g ml⁻¹)

Dissociation constant pKa) at 25 °C

Vapour pressure at 20 °C (mPa)

Henry's law constant at 25 °C (Pa m³ mol⁻¹)

Volatilisation as max % of applied dose lost

From plant surface

From soil surface

Maximum UV-vis absorption L mol⁻¹ cm⁻¹

Surface tension (mN m⁻¹)

Refractive Index

Environmental release

Degradation

Property

Value

Source; quality score; and other information

Interpretation

Soil degradation (days) (aerobic)

DT₅₀ (typical)

DT₅₀ (lab at 20 °C)

DT₅₀ (field)

DT₉₀ (lab at 20 °C)

DT₉₀ (field)

Note

Manure DT₅₀ (days)

Aqueous photolysis DT₅₀ (days) at pH 7

Value

Note

Aqueous hydrolysis DT₅₀ (days) at 20 °C and pH 7

Value

Note

Water-sediment DT₅₀ (days)

Water phase only DT₅₀ (days)

Sediment phase only DT₅₀ (days)

Air degradation

As this parameter is not normally measured directly, a surrogate measure is used: ‘Photochemical oxidative DT₅₀’. Where data is available, this can be found in the Fate Indices section below.

Decay in stored produce DT₅₀

Soil adsorption and mobility

Property

Value

Source; quality score; and other information

Interpretation

Linear

K_d (mL g⁻¹)

K_oc (mL g⁻¹)

Notes and range

Freundlich

K_f (mL g⁻¹)

K_foc (mL g⁻¹)

¹/_n

Notes and range

pH sensitivity

Fate indices

Property

Value

Source; quality score; and other information

Interpretation

GUS leaching potential index

Bio-concentration factor

BCF (l kg⁻¹)

CT₅₀ (days)

Known soil and groundwater metabolites

None

Other known metabolites

Metabolite name and reference

Aliases

Formation medium / Rate

Estimated maximum occurrence fraction

5-aminosalicylic acid
Note: Active substance

Colonic

ECOTOXICOLOGY

Terrestrial ecotoxicology

Property

Value

Source; quality score; and other information

Interpretation

Mammals - Acute oral LD₅₀ (mg kg⁻¹)

15600

Rat

Low

Mammals - Short term dietary NOEL

(mg kg⁻¹)

(ppm diet)

Mammals - Chronic 21d NOAEL (mg kg⁻¹ bw d⁻¹)

Birds - Acute LD₅₀ (mg kg⁻¹)

Birds - Short term dietary (LC₅₀/LD₅₀)

Birds - Chronic 21d NOEL (mg kg⁻¹ bw d⁻¹)

Earthworms - Acute 14 day LC₅₀ (mg kg⁻¹)

Earthworms - Chronic NOEC, reproduction (mg kg⁻¹)

Soil micro-organisms

Collembola

Acute LC₅₀ (mg kg⁻¹)

Chronic NOEC (mg kg⁻¹)

Non-target plants

Vegetative vigour ER₅₀ (g ha⁻¹)

Seedling emergence ER₅₀ (g ha⁻¹)

Honeybees (Apis spp.)

Contact acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

Oral acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

Unknown mode acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

Chronic

Notes

Bumblebees (Bombus spp.)

Contact acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

Oral acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

Mason bees (Osmia spp.)

Contact acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

Oral acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg bee⁻¹)

Other bee species (1)

Acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg insect⁻¹)

Mode of exposure

Other bee species (2)

Acute LD₅₀ (worst case from 24, 48 and 72 hour values - μg insect⁻¹)

Mode of exposure

Beneficial insects (Ladybirds)

Beneficial insects (Lacewings)

Beneficial insects (Parasitic wasps)

Beneficial insects (Predatory mites)

Beneficial insects (Ground beetles)

Aquatic ecotoxicology

Property

Value

Source; quality score; and other information

Interpretation

Temperate Freshwater Fish - Acute 96 hour LC₅₀ (mg l⁻¹)

Temperate Freshwater Fish - Chronic 21 day NOEC (mg l⁻¹)

Tropical Freshwater Fish - Acute 96 hour LC₅₀ (mg l⁻¹)

Temperate Freshwater Aquatic invertebrates - Acute 48 hour EC₅₀ (mg l⁻¹)

Temperate Freshwater Aquatic invertebrates - Chronic 21 day NOEC (mg l⁻¹)

Tropical Freshwater Aquatic invertebrates - Acute 48 hour EC₅₀ (mg l⁻¹)

Aquatic crustaceans - Acute 96 hour LC₅₀ (mg l⁻¹)

Sediment dwelling organisms - Acute 96 hour LC₅₀ (mg l⁻¹)

Sediment dwelling organisms - Chronic 28 day NOEC, static, water (mg l⁻¹)

Sediment dwelling organisms - Chronic 28 day NOEC, sediment (mg kg⁻¹)

Aquatic Plants (free-floating, fonds growth, fresh) - 7 day (mg l⁻¹)

Aquatic plants (rooted, growth rate, fresh) - 14 day (mg l⁻¹)

Algae - Acute (growth rate, fresh; mg l⁻¹)

Algae - Chronic (growth rate, fresh; mg l⁻¹)

Mesocosm study data

NOEAEC mg l⁻¹

Marine bivalves

HUMAN HEALTH AND PROTECTION

General

Property

Value

Source; quality score; and other information

Interpretation

Threshold of Toxicological Concern (Cramer Class)

Mammals - Acute oral LD₅₀ (mg kg⁻¹)

15600

Rat

Low

Mammals - Dermal LD₅₀ (mg kg⁻¹ body weight)

Mammals - Inhalation LC₅₀ (mg l⁻¹)

Other Mammal toxicity endpoints

Intravenous LD₅₀ = 1096 mg kg⁻¹

Mouse

Subcutaneous LD₅₀ = 3000 mg kg⁻¹

Mouse

ADI - Acceptable Daily Intake (mg kg⁻¹ bw day⁻¹)

ARfD - Acute Reference Dose (mg kg⁻¹ bw day⁻¹)

AAOEL - Acute Acceptable Operator Exposure Level (mg kg⁻¹ bw day⁻¹)

AOEL - Acceptable Operator Exposure Level - Systemic (mg kg⁻¹ bw day⁻¹)

Dermal penetration studies (%)

Dangerous Substances Directive 76/464

Exposure Routes

Public

Occupational

Mammalian dose elimination route and rate

Mainly eliminated in the urine either as free metabolites or as glucuronide conjugates

Health issues

Specific human health issues

Carcinogen

Genotoxic

Endocrine disruptor

; ; ; ;

No data found

Reproduction / development effects

Acetyl cholinesterase inhibitor

Neurotoxicant

No data found

Respiratory tract irritant

Skin irritant

Skin sensitiser

No data found

Eye irritant

Phototoxicant

No data found

General human health issues

May cause vomiting, loss of appetite and jaundice
May cause dry eyes
Studies suggest substance may act as a co-carcinogen

Handling issues

Property

Value and interpretation

General

Will emit toxic gases when heated to decomposition

CLP classification 2013

WHO Classification

Not listed (Not listed)

UN Number

Waste disposal & packaging

Shelf-life, storage, stability and reactivity

TRANSLATIONS

Language

Name

English

sulfasalazine

French

sulfasalazine

German

Sulfasalazin

Danish

Italian

Spanish

sulfasalazina

Greek

Polish

Swedish

Hungarian

Dutch

Norwegian

Record last updated:	12/09/2025
Contact:	aeru@herts.ac.uk
Please cite as:	Lewis, K.A., Tzilivakis, J., Warner, D. and Green, A. (2016) An international database for pesticide risk assessments and management. Human and Ecological Risk Assessment: An International Journal, 22(4), 1050-1064. DOI: 10.1080/10807039.2015.1133242