Melengestrol acetate

Melengestrol acetate	Last updated: 16/09/2025
(Also known as: MGA)

GENERAL INFORMATION

Description

Melengestrol Acetate is a progestogen-like compound used as a feed additive

Examples of veterinary uses

Used for growth promotion and suppression of estrus (heat

Examples of species treated

Cattle

Approval status

VMR 2013/2033 approval status (GB/UK)

Not approved

EU Regulatory approval status

Not approved

Chemical structure

Isomerism

Melengestrol acetate exhibits stereoisomerism due to the presence of multiple chiral centres within its steroidal framework. Specifically, it contains six stereocentres at positions C8, C9, C10, C13, C14, and C17 of the cyclopenta[a]phenanthrene ring system. The active form used in veterinary medicine is a single, well-defined stereoisomer, with the configuration: (8R,9S,10R,13S,14S,17R).

Chemical formula

C₂₅H₃₂O₄

Canonical SMILES

CC1=CC2C(CCC3(C2CC(=C)C3(C(=O)C)OC(=O)C)C)C4(C1=CC(=O)CC4)C

Isomeric SMILES

CC1=C[C@@H]2[C@H](CC[C@]3([C@H]2CC(=C)[C@@]3(C(=O)C)OC(=O)C)C)[C@@]4(C1=CC(=O)CC4)C

International Chemical Identifier key (InChIKey)

UDKABVSQKJNZBH-DWNQPYOZSA-N

International Chemical Identifier (InChI)

InChI=1S/C25H32O4/c1-14-11-19-20(23(5)9-7-18(28)13-21(14)23)8-10-24(6)22(19)12-15(2)25(24,16(3)26)29-17(4)27/h11,13,19-20,22H,2,7-10,12H2,1,3-6H3/t19-,20+,22+,23-,24+,25+/m1/s1

2D structure diagram/image available?

Yes

Cambridge Crystallographic Data Centre diagrams

Common Name	Relationship	Link
melengestrol acetate	-

General status

Veterinary substance type

Medical drug, Feed additive

Substance groups

Steroid; Synthetic progestin

Minimum active substance purity

Known relevant impurities

Substance origin

Synthetic

Mode of action

Progestins generally work by suppressing the secretion of GnRH, LH) and FSH by the pituitary gland. This suppression alters the menstrual cycle to suppress ovulation.

Molecular targets

[Gonadotropin-releasing hormone (GnRH), Antagonist], [Luteinizing hormone (LH), Antagonist], [Follicle-stimulating hormone (FSH) , Antagonist]

CAS RN

2919-66-6

EC number

220-859-7

CIPAC number

US EPA chemical code

PubChem CID

250948

Therapeutic Class

Genito urinary system and sex hormones; Progestogens

ATCvet Code

QG03DB91

Controlled Drug?

Regulation 37/2010 MRL Classification

Molecular mass

396.53

PIN (Preferred Identification Name)

IUPAC name

[(8R,9S,10R,13S,14S,17R)-17-acetyl-6,10,13-trimethyl-16-methylidene-3-oxo-1,2,8,9,11,12,14,15-octahydrocyclopenta[a]phenanthren-17-yl] acetate

CAS name

Forever chemical

Other status information

Relevant Environmental Water Quality Standards

Physical state

Off-white to pale yellow coloured powdery solid

Commercial

Property

Value

Availability status

1996, first approvals USA

Introduction & key dates

1960s, first synthesised; 1970s, livestock field trials; 1996, first aprovals USA

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

Zoetis Inc.

Example products using this active

MGA 200

Formulation and application details

Usually formulated as a Type A medicated feed additive

Commercial production

The production of melengestrol acetate involves a multi-step chemical synthesis starting from a steroidal precursor such as 17alpha-hydroxyprogesterone. The process begins with selective functional group modifications, including oxidation, methylation, and acetylation, to introduce the necessary substituents at the C6 and C17 positions. A key step is the acetylation of the 17alpha-hydroxyl group, forming the acetate ester that enhances the compound’s lipophilicity and metabolic stability.

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)

1.06

Low

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

Melting point (°C)

224

Boiling point (°C)

Degradation point (°C)

Flashpoint (°C)

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

9.55 X 10⁰³

Calculated

Log P

3.98

High

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

Substance may enter the environment via the urine of treated animals or by leaching from spilt medicated feed.

Degradation

Property

Value

Source; quality score; and other information

Interpretation

Soil degradation (days) (aerobic)

DT₅₀ (typical)

DT₅₀ (lab at 20 °C)

12.5

Non-persistent

DT₅₀ (field)

DT₉₀ (lab at 20 °C)

DT₉₀ (field)

Note

USEPA data: Lab studies DT₅₀ range 4.3-27.8 days, Soils=3

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)

779

K_oc (mL g⁻¹)

Notes and range

USEAPA: K_d range 549-1009 mL g⁻¹

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

2alpha-hydroxy-melengestrol acetate

Mammals

6-hydroxymethyl-melengestrol acetate

Mammals

ECOTOXICOLOGY

Terrestrial ecotoxicology

Property

Value

Source; quality score; and other information

Interpretation

Mammals - Acute oral LD₅₀ (mg kg⁻¹)

> 8000

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 dw soil)

> 2.0

Lumbricus terrestris

High

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

>= 2.0

Lumbricus terrestris

Moderate

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)

> 1.0

Carassius auratus

Moderate

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)

>= 2.0

Daphnia magna

Moderate

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

>= 2.0

Daphnia magna

Moderate

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)

> 8000

Rat

Low

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

Rat

Mammals - Inhalation LC₅₀ (mg l⁻¹)

Other Mammal toxicity endpoints

Intraperitoneal LD₅₀ > 2500 mg kg⁻¹

Mouse

Subcutaneous LD₅₀ > 5000 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

Extensively metabolised and mainly excreted in urine

Health issues

Specific human health issues (hazard-based)

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

No information available

Handling issues

Property

Value and interpretation

General

No information available

CLP classification 2013

WHO Classification

Not listed (Not listed)

UN Number

Waste disposal & packaging

Shelf-life, storage, stability and reactivity

TRANSLATIONS

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Name

English

melengestrol acetate

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Swedish

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Record last updated:	16/09/2025
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