Phenobarbital is available in tablet or capsule form, and as a liquid. All three forms are taken orally one to three times each day with or without food. When taken once a day, the drug is typically taken near bedtime.
The dosage is prescribed by a physician. Typically, the total daily dose ranges 30–120 mg. For treatment of seizures, the dosage can be 60–200 mg daily. The daily dosage for children is typically 3–6 mg per 2.2 lb (1 kg) of body weight.
Dosages should not be exceeded. It is also important to adhere to the proper timetable for use of the medication. Use of the drug should not be discontinued without consulting a physician.
Phenobarbital is potentially habit forming if taken over an extended period of time. When being prescribed to overcome insomnia, the drug should not be used for a period longer than two weeks. Furthermore, phenobarbital should not be taken in a dose that exceeds the prescribed amount. Ingestion of more than the recommended dosage can result in unsteadiness, slurred speech, and confusion. More serious results of overdose include unconsciousness and breathing difficulty.
Long-term use can lead to tolerance, making it necessary to take increased amounts of the drug to achieve the desired effect. This poses a risk of habitual use; however, it should be noted that people with seizure disorders seldom have problems with phenobarbital dependence. Nevertheless, with chemical dependency, symptoms of withdrawal from phenobarbital begin eight to 12 hours after the last dose, and progress in severity. Initial symptoms may include anxiousness, insomnia, and irritability. Twitching and tremors in the hands and fingers precludes increasing weakness, dizziness, nausea, and vomiting. Symptoms can sometimes become severe or life-threatening, with seizures, delirium, or coma.
While there is evidence of risk to a fetus, the benefits of phenobarbital for a pregnant woman can sometimes warrant its use. This must be determined by a physician.
Common side effects include drowsiness, headache, dizziness, depression, stomachache, and vomiting. More severe side effects include nightmares, constipation, and pain in muscles and joints. Side effects that require immediate medical attention occur rarely, and include
Phenobarbital can interact with a number of prescription and nonprescription medications including acetaminophen, anticoagulants such as warfarin, chloramphenicol, monoamine oxidase inhibitors (MAOIs), antidepressants, asthma medicine, cold medicine, anti-allergy medicine, sedatives, steroids, tranquilizers, and vitamins. Interactions with these medications can increase the drowsiness caused by phenobarbital. Decreased efficiency of anticoagulants can increase the risk of bleeding. Phenobarbital can also react with oral contraceptives, which can decrease the effectiveness of the birth control medication.
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The Epilepsy Foundation. 4351 Garden City Drive, Landover, MD 20785-7223. (800) 332-1000. <http://www.epilepsyfoundation.org/>.
Brian Douglas Hoyle, PhD