As previously reported, on August 1, 2026, Indivior Pharmaceuticals, Inc., a Delaware corporation (the "Indivior"), entered into an Agreement and Plan of Merger (the "Merger Agreement") with Supernus Pharmaceuticals, Inc., a Delaware corporation ("Supernus") and Artemis Merger Sub Inc., a Delaware corporation and a wholly owned subsidiary of Indivior ("Merger Sub"). The Merger Agreement provides that, upon the terms and subject to the conditions set forth in the Merger Agreement, Merger Sub will merge with and into Supernus (the "Merger"), with Supernus continuing as the surviving company and a wholly owned subsidiary of Indivior following the transaction. As a result of the Merger, Indivior will be renamed Supernus, Inc. (the "Combined Company").

In connection with the Merger, Indivior filed with the U.S. Securities and Exchange Commission (the "SEC") on August 28, 2026, and subsequently amended prior to effectiveness, a Registration Statement on Form S-4 (File No. 333-298637) to register the shares of Indivior's common stock to be issued in connection with the Merger (as amended, the "Registration Statement"). The Registration Statement includes a joint proxy statement of Indivior and Supernus (the "definitive joint proxy statement/prospectus"). The Registration Statement was declared effective by the SEC on September 11, 2026, and the definitive joint proxy statement/prospectus was first mailed to the stockholders of each of Indivior and Supernus. Indivior is filing this Current Report on Form 8-K (this "Supplement") to update and supplement the definitive joint proxy statement/prospectus.

Each of Indivior and Supernus will hold a special meeting of its stockholders on October 15, 2026 in connection with the Merger, as further described in the definitive joint proxy statement/prospectus.

Capitalized terms used but not otherwise defined in this Supplement have the meanings given to them in the definitive joint proxy statement/prospectus. This Supplement should be read in conjunction with the definitive joint proxy statement/prospectus, which should itself be read in its entirety. Except as specifically supplemented by the information contained in this Supplement, all information set forth in the definitive joint proxy statement/prospectus remains unchanged. To the extent that information in this Supplement differs from or updates information contained in the definitive joint proxy statement/prospectus, the information in this Supplement is more current and supersedes the different or inconsistent information contained in the definitive joint proxy statement/prospectus.

Background of the Supplemental Disclosures

Following the filing of the definitive joint proxy statement/prospectus, Supernus received demand letters from purported Supernus stockholders (collectively, the "Demand Letters") and two complaints were filed by purported stockholders of Supernus in the Supreme Court of the State of New York, County of New York (collectively, the "Complaints"). The Demand Letters and Complaints generally allege, among other things, that the definitive joint proxy statement/prospectus contains certain disclosure deficiencies and/or incomplete information regarding the Merger.

Supernus believes that the claims asserted in the Demand Letters and Complaints are without merit and that no supplemental disclosure to the definitive joint proxy statement/prospectus is or was required under any applicable law, rule or regulation. Supernus denies that it has violated any law or breached any duty to its stockholders, and denies all of the allegations in the Demand Letters and Complaints. However, solely to eliminate the burden, expense and uncertainty of litigation, to moot disclosure claims asserted in the Demand Letters and Complaints, and to avoid the risk that the Demand Letters or Complaints may delay or otherwise adversely affect the Merger, and without admitting any liability or wrongdoing, Supernus and Indivior have determined to voluntarily supplement the definitive joint proxy statement/prospectus with the disclosures set forth below (the "Supplemental Disclosures"). Nothing in this Supplement shall be deemed an admission of the legal necessity or materiality under any applicable law of any of the Supplemental Disclosures. To the contrary, Supernus and Indivior specifically deny that any additional disclosure is or was required.