Phenobarbital

Phenobarbital	Last updated: 16/09/2025
(Also known as: phenobarbitone; phenylethylbarbiturate; fenobarbitol)

GENERAL INFORMATION

Description

A barbiturate anticonvulsant veterinary drug

Examples of veterinary uses

Mainly used to treat seizure, as a sedative and as an antianxiety medication

Examples of species treated

Cats; Dogs; Ferrets; Horses; Sheep

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

None

Chemical formula

C₁₂H₁₂N₂O₃

Canonical SMILES

CCC1(C(=O)NC(=O)NC1=O)C2=CC=CC=C2

Isomeric SMILES

International Chemical Identifier key (InChIKey)

DDBREPKUVSBGFI-UHFFFAOYSA-N

International Chemical Identifier (InChI)

InChI=1S/C12H12N2O3/c1-2-12(8-6-4-3-5-7-8)9(15)13-11(17)14-10(12)16/h3-7H,2H2,1H3,(H2,13,14,15,16,17)

2D structure diagram/image available?

Yes

Cambridge Crystallographic Data Centre diagrams

Common Name	Relationship	Link
phenobarbital	-

General status

Veterinary substance type

Barbiturate; Anticonvulsant

Substance groups

Barbiturate

Minimum active substance purity

Known relevant impurities

Substance origin

Synthetic

Mode of action

A nonselective central nervous system depressant. It promotes binding to inhibitory gamma-aminobutyric acid subtype receptors, and modulates chloride currents through receptor channels.

Molecular targets

[Gamma-aminobutyric acid receptor subunit alpha-1, Potentiator], [Neuronal acetylcholine receptor subunit alpha-4, Antagonist], [Neuronal acetylcholine receptor subunit alpha-7, Antagonist], [Glutamate receptor 2, Antagonist], [Glutamate receptor ionotropic, kainate 2, Antagonist], [NMDA receptor, Antagonist], [Nuclear receptor subfamily 1 group I member 2, Activator]

CAS RN

50-06-6

EC number

200-007-0

CIPAC number

US EPA chemical code

PubChem CID

4763

Therapeutic Class

Antiepileptics, Barbiturates and derivatives

ATCvet Code

QN03AA02

Controlled Drug?

UK: Class B, Schedule 3; EU: UN71, Class IV

Regulation 37/2010 MRL Classification

Molecular mass

232.23

PIN (Preferred Identification Name)

5-ethyl-5-phenyl-1,3-diazinane-2,4,6-trione

IUPAC name

5-ethyl-5-phenyl-1,3-diazinane-2,4,6-trione

CAS name

Forever chemical

Other status information

Relevant Environmental Water Quality Standards

Physical state

White crystalline powder

Commercial

Property

Value

Availability status

Current

Introduction & key dates

1911, first synthesised

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

Vetoquinol UK Ltd
Chanelle Pharmaceuticals Manufacturing Ltd
Dechra Regulatory BV

Example products using this active

Epiphen Tablets
Epityl Flavoured Tablets
Phenoleptil Tablets

Formulation and application details

Available in a range of formulations including liquids for adding to feed and tablets for oral use

Commercial production

Phenobarbital is synthesised through a multi-step chemical process that begins with the preparation of key intermediates. One common route starts with benzoic acid and formic acid, which are condensed to form methyl phenylacetate. This intermediate is then reacted with thionyl chloride to produce phenylacetyl chloride, a reactive acyl compound. Next, phenylacetyl chloride is combined with isovaleryl urea under basic conditions to yield 5-phenyl-5-isovaleric acid, a precursor to the barbiturate ring. In the final step, this compound is reacted with ethanolamine in the presence of sodium hydroxide under reflux, forming the phenobarbital molecule. The product is then purified through acidification, solvent extraction, and distillation.

Impact on climate of production and use

GHG emissions are estimated to be 20–40 kg CO₂e per kg of phenobarbital, assuming standard industrial practices and solvent recovery systems.

ENVIRONMENTAL FATE

Property

Value

Source; quality score; and other information

Interpretation

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

34.8

Moderate

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

Melting point (°C)

174

Boiling point (°C)

Degradation point (°C)

Flashpoint (°C)

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

2.95 X 10⁰¹

Calculated

Log P

1.47

Low

Fat solubility of residues

Solubility

Data type

Density (g ml⁻¹)

Dissociation constant pKa) at 25 °C

7.3

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

1.557

Environmental release

Substance may enter the environment via the faeces of treated animals

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 metabolites

None

ECOTOXICOLOGY

Terrestrial ecotoxicology

Property

Value

Source; quality score; and other information

Interpretation

Mammals - Acute oral LD₅₀ (mg kg⁻¹)

162

Rat

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 dw soil)

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

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)

484

Pimephales promelas

Low

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)

1460

Daphnia magna

Low

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)

162

Rat

Moderate

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

Mammals - Inhalation LC₅₀ (mg l⁻¹)

Other Mammal toxicity endpoints

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

Pattern of use is unlikely to result in toxicological relevant exposure

Occupational

Pattern of use is unlikely to result in toxicological relevant exposure

Mammalian dose elimination route and rate

Eliminated mainly via kidney and faeces

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

CLP data - suspected carcinogen

Handling issues

Property

Value and interpretation

General

Combustible
Incompatible with strong oxidising substances

CLP classification 2013

Health: H360, H351, H317, H201

WHO Classification

Not listed (Not listed)

UN Number

UN2811

Waste disposal & packaging

Shelf-life, storage, stability and reactivity

Sensitive to prolonged exposure to light. Aqueous solutions are unstable.

TRANSLATIONS

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phenobarbital

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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