The company's Suboxone has been among the main treatments for addiction through a growing epidemic of opioid abuse in the United States. A film variant of Suboxone has lost more than half its market share to cheaper rivals in the last year.

Indivior's shares have also plunged, by nearly two-thirds in 2019, as it also battles a potentially crushing $3 billion (£2.3 billion) fine for illegally marketing Suboxone. The market share for branded Suboxone film was 24% at the end of last year, down from a 53% share at the end of 2018.

(Interactive graphic on Suboxone film - https://fingfx.thomsonreuters.com/gfx/editorcharts/INDIVIOR-RESULTS/0H001R8B8BVK/index.html)

Indivior, which gets the bulk of its sales from the United States, has gained from the government's efforts to combat the opioid epidemic and is focusing on newer drugs such as Sublocade, an injectable treatment, to boost sales.

The company, which also took a hit from higher marketing costs promoting Sublocade, said the new treatment's sales in 2020 were expected to be between $150 million and $200 million, while the company's total sales are expected to be in a range of $525 million to $585 million.

"Although we are optimistic about delivering on our strategic priorities in 2020, we of course recognise the legal uncertainties we face," Chief Executive Officer Shaun Thaxter said.

Indivior last year was indicted and accused of deceiving doctors and healthcare benefit programs in the United States into believing its Suboxone film, itself a form of opioid, was safer and less susceptible to abuse than similar drugs.

The company, spun-off from Reckitt Benckiser in 2014, said net loss was $55 million for the three months ended Dec. 31, compared to a profit of $24 million a year earlier.

Indivior's shares were down nearly 20% at 39.41 pence as at 0816 GMT and was the biggest loser on Britain's small-cap index.

(Reporting by Pushkala Aripaka in Bengaluru; Editing by Shounak Dasgupta)