Britain's Indivior is set to face cheap competition to its opioid addiction treatment Suboxone Film after losing a patent protection case, sending its shares more than 20 percent lower on Friday.
Indivior said the US District Court for the District of Delaware had found generic drugs firm Alvogen had not infringed three of the British firm's patents.
That weakens Indivior's defence against a company making and marketing a copycat version of the product that has accounted for as much as 80 percent of its revenue.
Indivior's shares fell to a six month low after the ruling and were down 20 percent to 325.7 pence at 0817 GMT.
The company said it would continue to vigorously defend its intellectual property and believed it had grounds to appeal.
The launch of a generic "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," Indivior said.
The company spun off from Reckitt Benckiser in 2014, has been treating addiction for more than two decades, initially selling tablets to help wean addicts off opioids including heroin and prescription painkillers.
Now its big seller is Suboxone Film, which patients place under their tongue or inside their cheek once a day to suppress cravings.
It launched a long-lasting Sublocade injection in the United States last month, which it hopes will become a blockbuster medicine, despite the fact initial sales are likely to be slow.
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