Valproic acid is a major anticonvulsant drug. It works by increasing the amount of a certain natural substance in the brain, hence it is used to treat mania (episodes of frenzied, abnormally excited mood), and people with bipolar disorder (manic-depressive disorder; a disease that causes episodes of depression, episodes of mania, and other abnormal moods). It is also used to prevent migraine headaches, but not to relieve headaches that have already begun. The drug is metabolized by the liver and is processed at a rate that varies from patient to patient and is affected by a patient’s age and the health of their liver. The most frequent side-effects are gastrointestinal disturbances, and hair loss. Symptoms of overdose may include: sleepiness, irregular heartbeat, coma (loss of consciousness for a period of time). Valproic acid levels must be maintained within a narrow therapeutic range. Too little and the patient may experience a recurrence of symptoms (seizures, mania, or migraines); too much and the patient may experience increased side effects. This test is usually performed to determine whether drug concentrations are in the therapeutic range.