Material Event (Form 8-K)

On October 24, 2025, Theriva Biologics, Inc. (the "Company") filed a prospectus supplement and the accompanying base prospectus (the "Prospectus Supplement") to its Registration Statement on Form S-3, as amended (File No. 333-279077), which Form S-3 was declared effective by the Securities and Exchange Commission (the "Commission") on September 25, 2024 (the "Registration Statement"), relating to the offer and sale of up to $4,019,597 of shares of the Company's common stock, par value $0.001 per share (the "common stock"), from time to time through or directly to A.G.P./Alliance Global Partners (the "Sales Agent") pursuant to the terms of that certain Amended and Restated At Market Issuance Sales Agreement, dated February 9, 2021, as amended by Amendment No. 1 thereto, dated May 3, 2021, as further amended by Amendment No. 2 thereto, dated May 2, 2024 (the "Sales Agreement").

Sales of the common stock, if any, under the Prospectus Supplement will be made by any method permitted that is deemed an "at the market offering" as defined in Rule 415(a)(4) promulgated under the Securities Act of 1933, as amended (the "Securities Act"). The Sales Agent is not required to sell any specific amount, but will act as our Sales Agent using commercially reasonable efforts consistent with its normal trading and sales practices. There is no arrangement for funds to be received in any escrow, trust or similar arrangement.

The Sales Agent will be entitled to compensation at a commission rate equal to up to 3.0% of the gross sales price per share sold. In connection with the sale of the common stock on our behalf, the Sales Agent will be deemed to be an "underwriter" within the meaning of the Securities Act and the compensation of the Sales Agent will be deemed to be underwriting commissions or discounts. We have also agreed to provide indemnification and contribution to the Sales Agent with respect to certain liabilities, including liabilities under the Securities Act.

The opinion of the Company's special Nevada counsel (Parsons Behle & Latimer) regarding the validity of the shares of common stock that will be issued pursuant to the Prospectus Supplement and Sales Agreement is filed herewith as Exhibit 5.1 and is incorporated by refence herein.

This Current Report on Form 8-K shall not constitute an offer to sell or the solicitation of an offer to buy any security nor shall there be any sale of these securities in any state in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such state.