* Q1 profit down 5 pct, revenue falls 3.8 pct
* Shares drops as much as 6 pct before clawing back
* U.S. market share of Suboxone Film drops to 55 pct
* Company to share Sublocade sales of data in Q2 (Adds shares, analyst comment, details on Sublocade)
By Justin George Varghese
May 2 (Reuters) - Britain's Indivior posted a drop in first-quarter profit, as its blockbuster treatment, Suboxone Film, lost market share in the U.S. due to stiff competition from generic versions, the maker of drugs that treat opioid addiction said.
Shares in Indivior, which was spun out from Reckitt Benckiser in 2014 and has risen about three-fold since, were down as much as 6 percent in early trading before clawing back most of it to trade 1.3 percent down on Wednesday.
Indivior's shares have taken a beating in recent months after losing a couple of legal cases in its battle to protect the patent of Suboxone Film, which generates 80 percent of its revenues.
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 2015, but Suboxone Film leads the market for a version which is placed under the tongue to suppress cravings.
Indivior is also in patent disputes with Allergan Plc's Actavis Laboratories, Endo International's Par Pharmaceutical and Teva Pharmaceutical Industries.
The market share of the company's best-selling opioid addiction drug fell to 55 percent year-to-date from 60 percent last year.
Indivior posted a 5 percent fall in pretax profit to $111 million, while revenue fell 3.8 percent to $255 million.
But the company said it continued to expect net revenue in the range of $1.13 billion to $1.17 billion and net income in the range of $290 million to $320 million in 2018.
Analysts at Morgan Stanley said they did "not expect material consensus estimate revisions for the year as both sales and costs will be phased toward the second half of 2018."
Indivior, which launched a once-a-month injectable drug to suppress opioid craving in the United States in February, hopes Sublocade will become a blockbuster medicine but expects initial sales to be slow.
The company said that "the launch to date is progressing well, with positive patient and physician feedback and strong initial payor coverage."
Sublocade, which could be launched in Canada, Australia and Europe from late 2019, is designed to eliminate the risk that treatment drugs could be missed or misused. Indivior said it planned to share sales data from the second quarter this year.
"We expect the early promise of the launch to translate into accelerating prescription trends in the coming quarters and our confidence that Sublocade will achieve at least $1 billion in peak annual net revenue has been further strengthened," Chief Executive Shaun Thaxter said.
Reporting by Justin George Varghese in Bengaluru Editing by Sunil Nair and Edmund Blaitr