Phenobarbital is approved in the United States by the Food and Drug Administration (FDA) to be used along with another seizure medicine (as "add-on" or "adjunctive" therapy) for partial and tonic-clonic seizures. It has been used alone for more than 80 years, however, to treat all kinds of seizures.
Phenobarbital usage outside of epilepsy was more widespread in the past. Today, it is still used in the treatment of tremor, insomnia, and narcotic withdrawal.
The most common side effect of phenobarbital is sleepiness or fatigue. Anyone taking phenobarbital for the first time should be very careful with driving, operating machinery, or any other dangerous activity. (However, most patients just starting phenobarbital for seizures probably have restrictions regarding these activities anyway.)
Other side effects include:
If these problems do not go away within several days, or are really bothersome, call the doctor, but never stop or reduce the medicine on your own. Sometimes the doctor can help with these side effects by changing the prescription:
Phenobarbital also has been found to reduce the effectiveness of oral contraceptives (birth control pills), so women who could become pregnant should discuss all birth control options with their doctor to choose the most appropriate one.
Some studies have found greater problems with behavior and thinking in children taking phenobarbital than in children taking other seizure medicines. Not all the results agree, however, perhaps because individual children are affected differently.
Taking extra calcium and vitamin D may help to prevent bone loss. If you have been taking phenobarbital for more than 5 years, it might be a good idea to have a bone density test, an easy, painless test. If the study shows thinning of your bones, your doctor may want you to see a specialist.
Long-term use of phenobarbital can lead to changes in the soft tissues of the body. Symptoms include pains in the joints or thickening in areas such as the palm or the bottom of the foot. If you notice any changes of this kind, you should report them to your doctor.
Phenobarbital belongs to a group of medicines that can be addictive. People who take it to treat epilepsy hardly ever become psychologically addicted, but they are likely to experience physical dependence. With physical dependence, withdrawal symptoms occur if the medication is stopped suddenly or if the amount taken is suddenly or greatly reduced. These withdrawal symptoms may include:
Withdrawal symptoms can be avoided or greatly reduced by lowering the amount used very slowly over a long time. This should be done under the guidance of a doctor.
Depression is a common side effect in patients taking phenobarbital. If you have past experience with depression, drug abuse, or suicidal tendencies, you should inform your doctor as Phenobarbital may not be the best option for you.
In general, it is wise to avoid alcohol while taking phenobarbital. Adults with a responsible approach to drinking (and who are taking a low to medium dose of phenobarbital) may want to talk to the doctor about the possibility of having an occasional drink or two. Most doctors recommend that phenobarbital should never be combined with alcohol, narcotics, tranquilizers, or antihistamines as the dangers of mixing phenobarbital and any of these substances are severe and could include coma and death.
Phenobarbital can be taken with other medicines, although with certain precautions. Phenobarbital can lower the level in the blood of several other seizure medicines making it possible that increased seizures can occur. These medicines include:
Medicines often used for other disorders are also affected. It is especially important to check with your doctor before taking any of these:
Phenobarbital reduces the effectiveness of oral contraceptives (birth control pills), hormonal injections and implants, therefore women using these methods of birth control should be especially cautious as unplanned pregnancies are more likely to occur. To prevent pregnancy, a woman taking phenobarbital may need to use a different type of birth control, raise the dosage of the contraceptive or increase the frequency of the implant or injection. She should tell both the doctor prescribing contraception and the doctor prescribing phenobarbital about the other medicine so that the appropriate doses can be chosen. Phenobarbital does not affect barrier types of birth control, like condoms, non-hormonal IUDs, and diaphragms.
Birth defects are probably a bit more common in the babies of women who take phenobarbital during pregnancy than in others. A large majority of women who take phenobarbital do have healthy, normal babies, however. The risk of defects is higher for women who take more than one seizure medicine and women with a family history of birth defects.
All women who are capable of becoming pregnant should take at least 0.4 mg (400 mcg) of the vitamin called folic acid every day because it helps to prevent specific birth defects called Neural Tube Defects. (The most well-known of these is spina bifida, in which the spinal cord is not completely enclosed.) Women with epilepsy should take between 1 and 4 mg of folic acid daily during their reproductive years. If the doctor thinks a woman is at especially high risk, the larger dose of folic acid—4 mg (4000 mcg) per day—may be recommended, beginning before the woman becomes pregnant.
Some babies born to mothers taking antiepileptic medications have had inadequate blood clotting within the first 24 hours after birth. It is often recommended that the mother be given 10 to 20 mg of vitamin K per day during the last month of pregnancy to prevent this problem.
About 50% of phenobarbital in the mother’s blood passes through breast milk, however how much of this actually enters the baby’s bloodstream is not known. Further, how phenobarbital affects the baby is unknown, although the most common side effects are sedation and poor suck. That said, it is strongly felt by neurologists and epileptologists that the benefits of breastfeeding largely outweigh the risks of exposing the baby to antiepileptic medications, including phenobarbital. As a matter of fact, the American Academy of Neurology and the American Epilepsy Society both recommend breastfeeding in women with epilepsy.
If you are a woman with epilepsy taking phenobarbital and would like to become pregnant, you should discuss this with your doctor.