Indivior's experimental drug to help fight America's growing opioid addiction crisis was approved by the U.S. Food and Drug Administration late on Thursday, boosting its sales prospects as competitors threaten revenues from an older product.

Shares in the London-listed company, which specializes in addiction treatment, were up 10.9 percent at 411 pence at 0818 GMT on Friday. The stock was the top gainer on the FTSE Midcap index.

"We believe this decision is key for Indivior's investment case ... A decent label and price should also help support near-term sentiment and a re-rating of the shares," said Jefferies analysts, who rate the stock as "buy".

The approval comes after an advisory committee to the FDA voted 18-1 that Indivior's injectable drug, known as RBP-6000 or Sublocade, could benefit addicts.

Sublocade is expected to be available to patients in the United States in the first quarter of 2018, Indivior said.

"We are encouraged by this approval and ... we expect the story to shift more and more towards Sublocade as it grows," Morgan Stanley analysts said.

United States is battling an opioid abuse epidemic that in 2015 killed 33,000 people. President Donald Trump recently declared the problem a national public health emergency.

Indivior's product is the first monthly injectable buprenorphine treatment. Indivior already sells Suboxone Film, a product that combines buprenorphine and nalexone and is placed under the tongue or inside the cheek.

In September, a U.S. court ruling cleared the way for a generic rival to Suboxone Film, knocking back Indivior's shares.

FDA Commissioner Scott Gottlieb recently announced the agency's plans to encourage widespread use among opioid addicts of less harmful opioids such as methadone and buprenorphine, the active ingredient in Sublocade.