September 15 (HealthDay News) -- The U.S. Food and Drug Administration has approved an extended release form of Keppra (levetiracetam) as an add-on treatment for people with epileptic seizures aged 16 and older, Belgian drug maker UCB said Monday.

"While many people with epilepsy are successfully treated with one or more of the currently available anti-epileptic drugs, a significant percentage still live with uncontrolled seizures or intolerable side effects," the company said in a news release.

Once-daily Keppra XR is expected to be available at U.S. pharmacies by the end of the month, UCB said. The original form was approved in 1999.

In clinical testing, the most common side effects included drowsiness and irritability, which also are common side effects of the drug's immediate-release form.

More than 3 million people in the United States have epilepsy, and more than 40 percent of people who take only a single anti-epileptic drug continue to have seizures, the company said.