Doctors have been able to study and observe the side effects of phenobarbital for many years. In some ways this is an advantage over new medicines. Many doctors no longer prescribe it unless other seizure medicines have failed, however, because of concern that its side effects often outweigh its benefits (except for certain groups such as very young children). It may be used more frequently in situations where cost is a critical issue, such as in developing countries.

The most common side effect of phenobarbital is sleepiness or fatigue. Be careful with driving, operating machinery, or any other dangerous activity until you know how you react to phenobarbital.

Other side effects include:

• depression
• hyperactivity (in children)
• trouble paying attention (in children and adults)
• dizziness
• memory problems
• decreased sexual interest
• for males, inability to have an erection
• slurred speech
• upset stomach
• anemia
• deficiency of the vitamin folic acid
• rash
• fever
• low calcium levels, bone loss

If you notice problems like any of these while you are taking phenobarbital, it's probably a good idea to discuss them with your doctor or nurse. You shouldn't stop taking phenobarbital or any other seizure medication without your doctor's advice.

Phenobarbital also has been found to reduce the effectiveness of oral contraceptives (birth control pills), so women who could become pregnant may want to consider another form of birth control.

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. If this easy, painless test shows thinning of your bones, your doctor may want you to see a specialist.

Be sure to read about the more serious side effects of phenobarbital. Serious problems are very rare but everyone who takes this medicine should be aware of them.

Long-term effects
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, tell the doctor.

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