Clenbuterol

Clenbuterol	Last updated: 07/09/2025
(Also known as: clen; angel dust; clenbuteroleum)

GENERAL INFORMATION

Description

A non-steroidal medication veterinary drug, usually used as the hydrochloride, which possesses some properties similar to those of anabolic steroids.

Examples of veterinary uses

Substance is used as a decongestant and bronchodilator to treat breathing disorders and as a smooth muscle relaxant. It is also used for increasing muscle mass.

Examples of species treated

Horses; Cattle

Approval status

VMR 2013/2033 approval status (GB/UK)

Approved - usually supplied as a prescription only medicine to be authorised by a veterinarian (POM-V)

EU Regulatory approval status

Approved

Chemical structure

Isomerism

Clenbuterol exhibits stereoisomerism, specifically chirality, due to the presence of a chiral centre at the carbon atom bonded to the hydroxyl group and the tert-butylamino side chain. This results in two enantiomers: the (R)-clenbuterol and (S)-clenbuterol forms. Studies have shown that the (R)-enantiomer is primarily responsible for the Beta2-adrenergic agonist activity, contributing to its bronchodilatory and thermogenic properties, while the (S)-enantiomer may exhibit reduced efficacy or different biological interactions2. Commercial clenbuterol is typically sold as a racemic mixture.

Chemical formula

C₁₂H₁₈Cl₂N₂O

Canonical SMILES

CC(C)(C)NCC(C1=CC(=C(C(=C1)Cl)N)Cl)O

Isomeric SMILES

International Chemical Identifier key (InChIKey)

STJMRWALKKWQGH-UHFFFAOYSA-N

International Chemical Identifier (InChI)

InChI=1S/C12H18Cl2N2O/c1-12(2,3)16-6-10(17)7-4-8(13)11(15)9(14)5-7/h4-5,10,16-17H,6,15H2,1-3H3

2D structure diagram/image available?

Yes

Cambridge Crystallographic Data Centre diagrams

Common Name	Relationship	Link
clenbuterol	-
S-clenbuterol	Isomer
R-clenbuterol	Isomer

General status

Veterinary substance type

Muscle relaxant, Bronchodilator agent, Thermogenic drug, Medicinal drug

Substance groups

Amine

Minimum active substance purity

Known relevant impurities

Substance origin

Synthetic

Mode of action

Steroid like action, induces activation of adenylate cyclase through the action of G proteins. A beta-2 selective adrenergic agonist

Molecular targets

[Beta-2 adrenergic receptor, Agonist], [Beta-1 adrenergic receptor, Agonist], [Beta-3 adrenergic receptor, Agonist], [Beta-nerve growth factor, stimulator], [Tumor necrosis factor]

CAS RN

37148-27-9

EC number

253-366-0

CIPAC number

US EPA chemical code

PubChem CID

Therapeutic Class

Genito urinary system & sex hormones: Other gynecologicals; Respiratory system: Drugs for obstructive airway diseases

ATCvet Code

QG02CA91; QR03CC13

Controlled Drug?

UK: Class C, Schedule 4 Pt2

Regulation 37/2010 MRL Classification

Allowed substance (Table 1: Bovine, Equidae)

Molecular mass

277.19

PIN (Preferred Identification Name)

IUPAC name

(RS)-1-(4-amino-3,5-dichlorophenyl)-2-(tert-butylamino)ethanol

CAS name

1-(4-amino-3,5-dichlorophenyl)-2-(tert-butylamino)ethan-1-ol

Forever chemical

Other status information

Relevant Environmental Water Quality Standards

Physical state

White to pale yellow crystalline powder

Related substances & organisms

clenbuterol hydrochloride

Commercial

Property

Value

Availability status

Current

Introduction & key dates

Late 1970s, developed in Germany

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

Boehringer Ingelheim Animal Health UK Ltd.
Le Vet Beheer B.V.

Example products using this active

Ventipulmin Granules
Dilaterol Syrup

Formulation and application details

Various formulations are available for both oral and intravenous administration

Commercial production

The synthesis of clenbuterol involves a multi-step organic process starting with 4-amino-3,5-dichlorobromoacetophenone as the key precursor. This compound is reacted with tert-butylamine in a mixture of tetrahydrofuran and ethanol under an inert nitrogen atmosphere, initially cooled in an ice bath and then brought to room temperature. The intermediate formed is then reduced using potassium borohydride, followed by further reaction in methanol to complete the transformation. After stirring for several hours, the reaction mixture undergoes solvent removal, aqueous quenching, and extraction with dichloromethane. The organic layers are dried and purified to yield clenbuterol.

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)

112

Moderate

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

Melting point (°C)

Boiling point (°C)

Degradation point (°C)

Flashpoint (°C)

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

8.71 X 10⁰²

Calculated

Log P

2.94

Moderate

Fat solubility of residues

Solubility

Data type

Density (g ml⁻¹)

Dissociation constant pKa) at 25 °C

17.84

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

73.38

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 metabolites

None

Known groundwater metabolites

None

Other known metabolites

None

ECOTOXICOLOGY

Terrestrial ecotoxicology

Property

Value

Source; quality score; and other information

Interpretation

Mammals - Acute oral LD₅₀ (mg kg⁻¹)

159

Rat as HCl salt

Moderate

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)

High (class III)

Mammals - Acute oral LD₅₀ (mg kg⁻¹)

159

Rat as HCl salt

Moderate

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

Mammals - Inhalation LC₅₀ (mg l⁻¹)

Other Mammal toxicity endpoints

Intravenous LD₅₀ = 27.6 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

May be absorbed through the skin

Mammalian dose elimination route and rate

Major fraction of the drug is excreted into the urine

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

Very toxic
Possible liver and cardiovascular toxicant
May cause muscle tremor, headache, dizziness and gastric irritation
May cause hypotension

Handling issues

Property

Value and interpretation

General

IMDG Transport Hazard Class 9

CLP classification 2013

WHO Classification

Not listed (Not listed)

UN Number

UN2811

Waste disposal & packaging

Packaging Group II (moderate danger)

Shelf-life, storage, stability and reactivity

TRANSLATIONS

Language

Name

English

clenbuterol

French

German

Danish

Italian

Spanish

Greek

Polish

Swedish

Hungarian

Dutch

Norwegian

Record last updated:	07/09/2025
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