Indivior said that as of Aug. 29 the U.S. Food and Drug Administration (FDA) had not announced any marketing authorisation to a generic Suboxone Film alternative.
"Indivior says it's hard to gauge the financial impact but its dominant market position could collapse in a matter of months," said Neil Wilson, senior market analyst at ETX capital.
Sales of Suboxone Film were higher in the first half of 2017 on the same period a year ago, but market share fell slightly to 59 percent from 61 percent. Generic rivals in tablet form are already on the market.
In September, 35 U.S. states and the District of Columbia filed a lawsuit against Indivior alleging that it tried to keep generic versions of a drug off the market.
The lawsuit by the states alleged that Indivior took steps to get patients to switch to the oral strip version of Suboxone.
Indivior said in its statement on Friday it would "continue vigorously defending its intellectual property."
Dr. Reddy's, whose shares were up 9 percent at 0832 GMT, bought the abbreviated new drug application (ANDA) for a generic rival treatment from Teva Pharmaceutical Industries in June 2016 for $70 million.
"Dr Reddy's still needs to obtain FDA approval. So even in the best case, launch by Dr. Reddy's is at least a year away," analysts at Credit Suisse wrote in a note.
Indivior said in June it had won a U.S. patent battle against Actavis and Par Pharmaceutical over Suboxone Film.