Phenobarbital has a long history of safe use. However, more serious adverse effects do occur once in a while and should be discussed with your doctor.

Barbiturates like phenobarbital can be addictive because of their sedative-hypnotic and anti-anxiety effects, but psychological dependence (a psychological craving after long-term use) is rare in people taking phenobarbital to treat epilepsy.

Physical dependence, on the other hand, is common. With physical dependence, a withdrawal syndrome occurs if the medication is stopped suddenly or if the dosage is sharply reduced. These withdrawal symptoms may include:

• anxiety
• increased heart rate
• sweating
• confusion
• seizures
• death (very rarely)

If you do become dependent, it is important for your doctor to withdraw you from the phenobarbital gradually to avoid withdrawal symptoms.

Depression is also a side effect, so if you have any history of depression, drug abuse, or suicidal tendencies, be especially careful with this medicine.

Women who may become pregnant should be aware that studies have suggested a connection between phenobarbital and a higher-than-expected incidence of birth defects. There also is a danger to the baby from withdrawal symptoms if the mother has taken phenobarbital in the last few months of pregnancy, since barbiturates cross the placenta into the baby. Communicate with your doctor about the potential effects of phenobarbital if you are pregnant or plan to become pregnant. Since phenobarbital reduces the effectiveness of oral contraceptives (birth control pills), you should be especially cautious.

In general, it is wise to avoid alcohol while taking phenobarbital. Adults who take a responsible appproach to drinking alcohol and who are taking a low to medium dosage of phenobarbital on a long-term basis should talk with the doctor about the possibility of having small quantities of alcoholic beverages. Some doctors do recommend that a barbiturate like phenobarbital should never be combined with any other central nervous system depressants—not only alcohol but also narcotics, tranquilizers, and antihistamines. You might lapse into a coma or die if, for example, you drank a large amount of alcohol in one night while taking an ample dose of phenobarbital.

Finally, it's not unusual for phenobarbital to make people feel sleepy or uncoordinated. If you've just started taking phenobarbital or have just had your dosage increased, be careful about doing things that could be dangerous (like driving or operating machinery) until you know how it will affect you.

Reviewed February 2004 by Steven C. Schachter, MD, epilepsy.com Editorial Board.