The barbiturates are nonselective central nervous system (CNS) depressants primarily used as sedative-hypnotics. In sub-hypnotic doses, they are also used as anticonvulsants. The barbiturates and their sodium salts are subject to control under the Federal Controlled Substances Act.
Phenobarbital is a barbituric acid derivative for oral administration and occurs as a white, odorless, slightly bitter powder that is soluble in chloroform, freely soluble in alcohol or ether, and slightly soluble in water. Its saturated solution has a pH of about 5.6. Chemically, it is 5-ethyl-5-phenylbarbituric acid with the molecular formula C12H12N2O3 (232.24).
Each Phenobarbital Tablet, USP contains 15 mg, 16.2 mg, 30 mg, 32.4 mg, 60 mg, 64.8 mg, 97.2mg, or 100 mg of Phenobarbital, USP.
Before taking this medicine
You should not take phenobarbital if you are allergic to phenobarbital or other barbiturates (Nembutal, Seconal, and others), or if you have:
- severe asthma, chronic obstructive pulmonary disorder (COPD), or other breathing disorder;
- a personal or family history of porphyria (a genetic enzyme disorder that causes symptoms affecting the skin or nervous system);
- severe liver disease; or
- a history of addiction to phenobarbital or similar medicines (Valium, Xanax, Ativan, and others).
Follow your doctor’s instructions about taking seizure medication if you are pregnant. Seizure control is very important during pregnancy, and having a seizure could harm both mother and baby. Do not start or stop taking this medicine without your doctor’s advice, and tell your doctor right away if you become pregnant.
Phenobarbital can make birth control pills less effective. Ask your doctor about using a non-hormonal birth control (condom, diaphragm with spermicide) to prevent pregnancy. It is not known whether phenobarbital passes into breast milk or if it could harm a nursing baby. Tell your doctor if you are breast-feeding a baby.
Phenobarbital, the longest-acting barbiturate, is used for its anticonvulsant and sedative-hypnotic properties in the management of all seizure disorders except absence (petit mal).
Mechanism of action
Phenobarbital acts on GABAA receptors, increasing synaptic inhibition. This has the effect of elevating seizure threshold and reducing the spread of seizure activity from a seizure focus. Phenobarbital may also inhibit calcium channels, resulting in a decrease in excitatory transmitter release. The sedative-hypnotic effects of phenobarbital are likely the result of its effect on the polysynaptic midbrain reticular formation, which controls CNS arousal.
Adults (seizure prevention): The recommended adult dose of phenobarbital to prevent seizures ranges from 30 mg to 250 mg daily in a single dose or in divided doses. The maximum dose is 600 mg daily.
Children (seizure prevention): The recommended children’s dose ranges from 1 mg to 6 mg per kilogram daily, in a single dose or in divided doses.
Adults (treatment of insomnia/sedative): The recommended adult dose of phenobarbital for insomnia ranges from 100 mg to 200 mg taken at bedtime. The recommended adult dose as a sedative ranges from 30 mg to 90 mg daily in 2 or 3 divided doses.
Children (sedative): The recommended sedative dose for children is 2 mg per kilogram of body weight, taken 3 times daily.
Many things can affect the dose of medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose different from the ones listed here, do not change the way that you are taking the medication without consulting your doctor.
This medication may be habit-forming if taken for long periods of time. You may experience withdrawal effects if you stop taking this medication suddenly after extended use. If you plan on stopping the medication, your doctor may want you to reduce the dose gradually to reduce the severity of withdrawal effects.
It is important that this medication be taken exactly as prescribed by your doctor. If you have been taking this medication on a regular basis, do not stop taking this medication without talking to your doctor first, and do not increase the dose of this medication without first talking with your doctor.
If you miss a scheduled dose, take it as soon as possible and continue on with your regular schedule. If it is almost time for your next dose, skip the missed dose and continue on with your regular dosing schedule. Do not take a double dose of this medication. If you are not sure what to do after missing a dose, contact your doctor or pharmacist for advice.
Do not dispose of medications in wastewater (e.g. down the sink or in the toilet) or in house hold garbage. Ask your pharmacist how to dispose of medications that are no longer needed or have expired.
Do not use phenobarbital if you:
- are allergic to phenobarbital or any ingredients of this medication
- have alcoholism
- have drug dependence
- have kidney impairment
- have liver impairment
- have porphyria
- have reduced lung function
- have severe respiratory depression
- have sleep apnea
- have suicidal tendencies
- have uncontrolled pain
Phenobarbital side effects
Phenobarbital may cause a severe allergic reaction. Stop taking phenobarbital and get emergency medical help if you have signs of an allergic reaction: hives; difficult breathing; swelling of your face, eyes, lips, tongue, or throat.
Call your doctor at once if you have:
- weak or shallow breathing;
- unusual pain anywhere in your body (especially in the neck, shoulder, or arms);
- a red blood cell disorder–pale skin, muscle weakness, diarrhea, weight loss, rapid heart rate, tongue swelling, numbness or tingling in your hands or feet, feeling short of breath; or
- severe skin reaction–fever, sore throat, swelling in your face or tongue, burning in your eyes, skin pain followed by a red or purple skin rash that spreads (especially in the face or upper body) and causes blistering and peeling.
Common side effects may include:
- drowsiness, lack of energy;
- dizziness or spinning sensation;
- depressed mood;
- feeling restless or excited (especially in children or older adults);
- drunk feeling; or “Hangover” effect (drowsiness the day after taking phenobarbital).
This is not a complete list of side effects and others may occur
- Avoid alcohol.
- Avoid excessive quantities of coffee or tea (Caffeine).
- Increase dietary intake of magnesium, folate, vitamin B6, B12, and/or consider taking a multivitamin.
- Take on an empty stomach for quicker absorption
Interactions with other drugs
Taking phenobarbital with other drugs that make you sleepy or slow your breathing can cause dangerous or life-threatening side effects. Ask your doctor before taking phenobarbital with a sleeping pill, narcotic pain medicine, muscle relaxer, or medicine for anxiety, depression, or seizures.
Other drugs may interact with phenobarbital, including prescription and over-the-counter medicines, vitamins, and herbal products. Tell each of your health care providers about all medicines you use now and any medicine you start or stop using.