Two senators have launched an investigation into practices by the US Food and Drug Administration, amid allegations that pharmaceutical companies eager for a chunk of the US$9 billion painkiller market may have paid to influence decisions.

"It's almost unheard of," said Andrew Kolodny, president of the group Physicians for Responsible Opioid Prescribing.

"For FDA to approve a drug that is going to make a serious problem worse, it is pretty shocking."

Zohydro was approved in October, even though a panel of FDA-convened experts voted 11-2 against the move. The FDA is not obliged to follow the advice of its advisory committees, but it typically does.

An FDA spokesman said the decision was made "after careful consideration" and "the product's benefits outweigh its risks when used as intended".

Zohydro contains pure hydrocodone in a range of doses, including time-release options that are much stronger than competitor products.

It does not contain acetaminophen, which can cause liver damage and death in high doses.

The drugmaker, Zogenix, said that the "acetaminophen-free formulation of extended release hydrocodone is an important therapeutic option for certain chronic pain patients".

Musician and financial adviser Steve Rummler was one of the 100 million people in the United States who suffer from chronic pain.

He was prescribed narcotic painkillers in 2005 for a lingering back injury.

"The doctor kept increasing the amounts that he would give to Steve," his mother, Judy Rummler, said.

By 2009, Steve's family learned he was seeking painkillers from multiple sources. He tried to learn other ways of coping. He tried rehab for addiction.

When his last refills ran out, he bought some heroin and overdosed on July 1, 2011, at the age of 43.

A total of 100 people in the United States die each day from drug overdose, according to the Centres for Disease Control and Prevention.