Phenobarbital is a barbiturate drug that slows the activity of the brain and nervous system. According to Drugs.com, phenobarbital is used to treat or prevent seizures. Additionally, this medication can be used to treat insomnia, or can be used as a sedative before surgery. As with all medications, a patient should be aware of the possible long-term side effects, and should know what to do if he experiences them.

Drowsiness

Drowsiness is the most common side effect associated with this medication, states Medline Plus of the National Institutes of Health. Drowsiness occurs because phenobarbital lowers the threshold for nerve signal conduction in the brain that causes you to be awake. While effective in treating insomnia, sleepiness may affect the patient during the day; therefore, a patient should not drive or operate heavy machinery until he knows how his body responds to this medication. Additionally, a patient should let his doctor know if drowsiness is affecting his daily life. He should not immediately stop taking phenobarbital as this can increase his risk of having seizures. Instead, he should see his doctor to have his dose reduced, or have the doctor prescribe a different medication to treat his condition.

Changes in Soft Tissue

According to Epilepsy.com, phenobarbital can lead to soft tissue changes. This includes joint pain, heel and knuckle pads, frozen shoulder, Dupuytren's contractures, fibrous tumors on the bottom of the foot, and Peyronie's disease, which is a hardening of a portion of the penis. Early recognition of these long-term effects can prevent further soft tissue changes. A doctor will likely prescribe a different medication if these long-term effects are reported.

Dementia

According to the journal "Acta Neurologica Scandinavica," phenobarbital may cause dementia in certain patients. This occurs because phenobarbital may be toxic to the portion of the brain that is responsible for creating and storing memories. A patient or his family may notice a decline in recent or long-term memory and unusual behavior. Early recognition of dementia and treatment may lead to full recovery. The doctor will likely reduce the dosage, or prescribe a different medication to reduce symptoms.