(Reuters) – Mallinckrodt Plc said on Tuesday it has received a subpoena from the U.S. Securities and Exchange Commission for documents related to the drugmaker’s lawsuit against the U.S. Department of Health and Human Services (HHS).

The lawsuit against the HHS and the Centers for Medicare and Medicaid Services was filed in May over changes in the calculation of Medicaid discounts for Acthar gel, the company’s biggest money-spinner. (http://bit.ly/2PNaeFz)

Changes to the Medicaid rebate calculations could result in a near 10% hit to annual sales of Acthar, its costly treatment for multiple sclerosis and a rare infant seizure disorder, the company said in May.

The drugmaker on Tuesday reported a 20.8% fall in sales of Acthar gel for the quarter ended Sept. 27 due to the continued reimbursement challenges. Mallinckrodt, among opioid manufacturers facing thousands of lawsuits seeking to hold them responsible for fueling an addiction crisis in the United States, is also grappling with an industrywide decline in generic drug prices and competition to two of its important drugs.

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Against this backdrop, the company in August suspended plans to spin off its specialty generics unit, citing opioid litigation uncertainties, and also warned Acthar gel revenue was unlikely to exceed $1 billion this year.

Shares of the company, which in September agreed to sell its contract manufacturing unit to reduce debt, have slumped nearly 80% so far this year.

(Reporting by Manojna Maddipatla in Bengaluru; Editing by Sriraj Kalluvila)