[424B5] Theriva Biologics, Inc. Prospectus Supplement (Debt Securities)

Rhea-AI Filing Summary

Theriva Biologics (TOVX) amended certain existing warrants, reducing the exercise price from $2.00 to $0.54 per share for warrants held by certain holders covering up to 1,345,000 shares of common stock. The change was executed on October 16, 2025 and disclosed via this amendment to a prior prospectus supplement tied to the company’s effective S-1.

The filing updates the earlier prospectus to reflect the revised pricing terms for these “Existing Warrants.” This adjustment affects only the warrant exercise price mechanics and does not alter the previously described securities from the earlier offering materials. Theriva’s common stock closed at $0.84 on October 15, 2025 on the NYSE Capital Market, providing context for the new exercise price.

Insights

Capital markets analyst

Warrant repricing to $0.54 may facilitate future exercises; neutral.

Theriva reduced the exercise price on certain Existing Warrants from $2.00 to $0.54 per share, covering up to 1,345,000 underlying shares. This action aligns the strike closer to the $0.84 stock close on October 15, 2025, which can make exercising more feasible for holders when permitted.

While the filing does not state proceeds or timing, a lower strike typically increases the likelihood of exercises when in-the-money, which could provide cash to the company upon any cash exercises. Any share issuance from exercises would depend on holder decisions and applicable terms.

The amendment updates prior offering materials and is administrative in nature. Actual impact will depend on whether and when holders choose to exercise under the revised $0.54 price.

Filed pursuant to Rule 424(b)(5)

Registration No. 333-282024 

Amendment No. 1 to
PROSPECTUS SUPPLEMENT NO. 1

(to the prospectus dated September 25, 2024)

Theriva Biologics, Inc.

1,345,000 Shares of Common Stock

This prospectus supplement updates, amends and supplements the prospectus contained in our Registration Statement on Form S-1, as amended, effective as of September 25, 2024 (as supplemented or amended from time to time, the “Prospectus”) (Registration No. 333-282024) relating to the offer and sale of up to 918,600 shares of common stock, par value $0.001 per share (the “Common Stock”), of Theriva Biologics, Inc. (the “Company”), 1,428,600 shares of Common Stock issuable upon the exercise of 1,428,600 common warrants to purchase shares of Common Stock (the “Existing Warrants”), 510,000 pre-funded warrants to purchase up to 510,000 shares of common stock, and 1,938,600 shares of Common Stock underlying the Existing Warrants and pre-funded warrants purchased by investors in a best efforts public offering that was consummated on or about September 27, 2024. This Amendment No. 1 to the Prospectus Supplement No. 1, filed on October 16, 2025 (as amended, the “Prospectus Supplement”), in order to amend the disclosure below under “Amendments to Existing Warrants” regarding the number of Existing Warrants being amended. Capitalized terms used in this Prospectus Supplement and not otherwise defined herein have the meanings specified in the Prospectus.

This Prospectus Supplement is not complete without the Prospectus. This Prospectus Supplement should be read in conjunction with the Prospectus, which is to be delivered with this Prospectus Supplement, and is qualified by reference thereto, except to the extent that the information in this Prospectus Supplement updates or supersedes the information contained in the Prospectus. Please keep this Prospectus Supplement with your Prospectus for future reference.

Our shares of Common Stock are listed on the NYSE Capital Market under the symbol “TOVX”. On October 15, 2025, the closing price for our shares of Common Stock on the NYSE Capital Market was $0.84 per share.

Investing in our securities involves a high degree of risk. Before deciding whether to invest in our securities, you should consider carefully the risks and uncertainties under the heading “Risk Factors” beginning on page 6 of the Prospectus.

Neither the SEC nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of the Prospectus or this Prospectus Supplement. Any representation to the contrary is a criminal offense.

AMENDMENTS TO EXISTING WARRANTS

This Prospectus Supplement is being filed to disclose the following:

On October 16, 2025, we entered into an amendment with certain holders of Existing Warrants to purchase up to 1,345,000 shares of Common Stock held in the aggregate by such holders in order to amend the exercise price under such Existing Warrants from $2.00 to $0.54 per share.

The date of this Prospectus Supplement is October 16, 2025.

FAQ

What did Theriva Biologics (TOVX) change in this amendment?

It reduced the exercise price of certain Existing Warrants from $2.00 to $0.54 per share.

How many shares are covered by the amended warrants for TOVX?

The amendment applies to Existing Warrants covering up to 1,345,000 shares of common stock.

When was the warrant amendment executed?

The company entered into the amendment on October 16, 2025.

What is the context of this update relative to prior filings?

It updates a prior prospectus supplement associated with an effective Form S-1 to reflect the revised warrant terms.

What was TOVX’s stock price near the time of the amendment?

Theriva’s common stock closed at $0.84 on October 15, 2025 on the NYSE Capital Market.

Does this amendment indicate SEC approval of the securities?

No. The filing states regulators have neither approved nor disapproved these securities or the disclosure.