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Central Nervous System Drugs
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Phenobarbital Sodium


Luminal Sodium


Tablets: 15 mg, 16 mg, 30 mg, 60 mg, 100 mg
Elixir: 15 mg/5mL, 20 mg/5mL
Injectable: 30 mg/mL


An antileptic agent, phenobarbital is of the barbiturate class of drugs. It is a central nervous system depressant, and works by facilitating the action of GABA (a neurotransmitter), suppressing the spread of seizure activity, and increasing seizure threshold.

Phenobarbital is one of the least toxic of the antileptics. It has a slow onset and long duration of action. Although it is the least toxic of this classification of drugs, it is considered to have a narrow margin of safety when used in rats and mice due to its link to hepatic and thyroid changes over lifetime use. However, in many of these studies, doses were given at toxic levels to determine what effect it might have on various organs when used in humans.

Phenobarbital is distributed widely throughout the body. Its rate of distribution into the central nervous system is slower than other barbiturates due to its low lipid soluability. It is also slowly absorbed in the GI tract.

The class of barbiturates can cause fetal damage and should not be used during pregnancy if possible.

Phenobarbital is metabolized in the liver and excreted partially unchanged in the kidneys. It has also been shown to be excreted in breast milk.


Used to treat generalized tonic-clonic, or focal type seizures.

Drug Interactions or Contraindications

An increased risk of hepatotoxicity can occur when acetaminophen (e.g.,Tylenol) is given with long term (chronic) use of phenobarbital.

The effects of phenobarbital may increase if used with other CNS depressants such as:

  • antihistamines
  • diazepam (valium)
  • opiate narcotics
  • phenothiazines (e.g., phenergan)
  • valproic acid (e.g.,Depakote

Phenobarbital may lower serum concentrations of the following drugs thereby decreasing their effects:

  • anticoagulants
  • Beta-Blockers (e.g., atenolol)
  • chloramphenicol
  • corticosteroids
  • doxycycline
  • griseoflvin
  • metronidazole
  • theophylline

Adverse Reactions

CV:  slow heartrate (bradycardia), low blood pressure (hypotension). CNS:  lethargy, sedation, ataxia, or paradoxical excitement. Often dose related, these signs may subside with continued therapy. Monitor closely.

Respiratory:  breathing depressed.

GI:  polydipsia (increase thirst)

GU:  polyuria (increased urine output)

Skin:  abscess or necrosis at injection site.

Dosage Recommendations

Initial dose is 2 mg/kg, PO, BID (twice a day or every 12 hours), and gradually increasing to 4 mg/kg , PO, BID (twice a day or every 12 hours), depending on effectiveness in controlling seizure activity. 16

Instruct pet owner to observe their pet rat closely, and report increased lethargy, increased excitability, bleeding, or changes in frequency, duration, and appearance of seizures.


  • May be combined with potassium bromide and / or gabapentin where seizures are refractory to phenobarbital treatment alone; or where the reduction of phenobarbital is desired due to side effects.
    Please note that it is imperative to discuss the changing, or adding, of any medications during your rat’s treatment with your veterinarian.
  • Do not use solutions that are discolored or that contains precipitates.
  • Do not skip doses, or stop drug suddenly, as this may initiate a seizure or cause seizures to worsen. Consult with veterinarian.
  • Encourage increase fluid intake by providing fresh fruit that is juicy.
  • Give with food, if possible, to decrease chance of distress to GI tract.
  • Injections may cause sterile abscesses with sloughing of tissue. Oral dosing is the preferred method for rats.

  • Kolaja, K., Stevenson, D., Johnson, J., Walborg, E., & Klaunig, J. (1996). Subchronic effects of dieldrin and phenobarbital on hepatic DNA synthesis in mice and rats. Fundam Appl Toxicol, 29(2), 219-28. Retrieved December 15, 2008, from the PubMed database.

Posted on August 3, 2004, 17:32, Last updated on November 9, 2015, 18:36 | Central Nervous System Drugs

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