India's Dr Reddy's Laboratories and U.S.-based Mylan NV received FDA approval on Thursday for a generic version of Suboxone Film, which generates 80 percent of Indivior's revenue.

Generic rivals in tablet form are already on the U.S. market, which is grappling with an opioid addiction epidemic that killed 33,000 people in 2015, but Suboxone Film leads the market with its version which is placed under the tongue to suppress cravings.

Dr Reddy's said it would launch its new drug regardless of the legal squabble.

"We are surprised by Dr. Reddy's decision to launch "at risk" given the ongoing litigation and associated significant risk to them of substantial economic damages if, as we believe, we eventually prevail in protecting the Suboxone Film patent estate," Indivior's Chief Executive Shaun Thaxter said.

Indivior warned, however, that Dr.Reddy's launch could "potentially result in a rapid and material loss of market share for Suboxone Film in the U.S., an effect that could occur within months of a successful launch of a generic film alternative."

The market share of the drug had declined to 55 percent by the end of April from 60 percent last year, the British firm said last month.

Indivior stuck to its 2018 revenue guidance but said it would revisit the forecast if Dr. Reddy's launches the cheaper drug. Indivior had forecast net revenue of $1.13 billion to $1.17 billion and net income of $290 million to $320 million.

Indivior said it was considering a potential launch of its own generic and operational cost cuts in response.

Brokerage Jefferies warned that Indivior's 2018 earnings per share could be hit by as much as 50 percent if Dr. Reddy's has sufficient inventory and launches by July 1.

The company said its contingency plans would also focus on supporting and optimising the launch of Sublocade, a once-a-month injectable drug to suppress opioid craving that it launched in the United States in February.

However, the company said in February that initial sales could be slow.

The FDA's decision on Thursday wiped out about a quarter of Indivior's market value, which is currently $4.82 billion (3.6 billion pounds). Dr. Reddy's shares closed up 3.5 percent.

Shares of Indivior, which was spun out from Reckitt Benckiser in 2014 and has tripled in value even after this week's fall, have taken a beating in recent months after losing legal cases to protect the patent of Suboxone Film.

It is also in patent disputes with Allergan Plc Actavis Laboratories, Endo International's Par Pharmaceutical and Teva Pharmaceutical Industries.

(Reporting by Justin George Varghese in Bengaluru; editing by Elaine Hardcastle)

By Justin George Varghese