The FDA has approved a drug currently used for restless leg syndrome to treat a common and painful complication of shingles, according to the medication's manufacturers.

Horizant (gabapentin enacarbil) has been approved to treat postherpetic neuralgia (PHN) in adults.

PHN is a painful syndrome that can lead to disability in some people. It occurs after an outbreak of herpes zoster, more commonly known as shingles.

Researchers say about 1 million people in the U.S. develop shingles each year. Of these, about 10% will go on to develop PHN.

PHN causes pain in the area affected by shingles, often in the torso, and can last for months or even years after the initial infection.

Shingles Slideshow: Photos of Shingles (Herpes Zoster)
New Use for Drug

Horizant was approved in 2011 to treat restless leg syndrome.

In approving Horizant to treat PHN, the FDA evaluated the safety and effectiveness of the drug in three clinical trials involving 574 people.

Researchers don't know exactly how Horizant works to reduce PHN-associated symptoms, but studies have shown that gabapentin can help relieve nerve-related pain.

The most common side effects of Horizant are dizziness, sleepiness, and headaches.

The drug may also impair a person's ability to drive a car. People taking Horizant should not drive a car or operate heavy machinery until they have enough experience with the drug to determine whether their driving ability is affected.

Horizant is a slightly less active form of the antiepileptic drug gabapentin. This type of antiepileptic drug increases the risk of suicidal thoughts and behaviors.

Horizant also increases these risks, and people taking the drug should talk to their health care provider about any new or worsening depression or suicidal thoughts.

The drug was developed by XenoPort Inc. and is marketed by GlaxoSmithKline.