(Adds details, context on drug, prior investigations, shares, analyst comment)
May 31 (Reuters) - Britain's Indivior has been subpoenaed by the California Department of Insurance over its blockbuster opioid addiction treatment, Suboxone Film, which is rapidly losing market share due to stiff competition from generic versions.
Indivior said on Thursday the agency, which is charged with oversseing insurance regulations, requested documents from its U.S. unit relating to the preparation of the drug, manufacturing records, and the potential to develop dependency on the treatment.
The investigation is one of several state and federal probes into the company's drug, which commands more than half the market and is suspected of being too aggressively promoted, delaying the market entry of generic rivals, and of being overprescribed by some doctors.
Shares in Indivior, which has received a number of subpeonas from California over the last two years, fell 2.3 percent to 467.5 pence by 0813 GMT.
Brokerage Jefferies called the development "additive but not game changing", estimating that the state subpoenas accounted for the smallest portion of potential litigation settlements, with a Department of Justice probe accounting for the majority.
It kept its "buy" rating on the stock.
Indivior in February raised its provisions for investigative and antitrust litigation by $185 million to $438 million.
The company, which was spun out from Reckitt Benckiser in 2014, said it intends to comply with the latest subpoena and will cooperate in "any related government inquiry".
Generic rivals to Suboxone in tablet form are already on the market in the United States, 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 said on May 2 Soboxone's market share had fallen to 55 percent from 60 percent a year earlier. (Reporting By Justin George Varghese in Bengaluru Editing by Georgina Prodhan)