Company: SNY
Filing Date: 2025-10-27
Form Type: 424B5
Source: 0001193125-25-250786
Chunk: 6

Company: Sanofi
Filing Date: 2025-10-27
Form: 424B5
Chunk 6
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 the time limits prescribed under, and are subject to limitations and defenses under, applicable Canadian securities legislation. The
purchaser should refer to the applicable Canadian securities legislation or consult with a legal advisor.

S-3

INCORPORATION OF INFORMATION WE FILE WITH THE SEC We have filed with the U.S. Securities and Exchange Commission (the “SEC”) a registration statement on Form F-3relating to the notes covered by this prospectus supplement and the attached prospectus. This prospectus supplement and the attached prospectus are part of that registration statement and do not contain all the information in the registration statement. Whenever a reference is made in this prospectus supplement and the attached prospectus to a contract or other document of Sanofi, the reference is only a summary. You should refer to the exhibits that are a part of the registration statement for a copy of the contract or other document. You may review a copy of the registration statement at the SEC’s public reference room in Washington, D.C., as well as through the SEC’s Internet site at http://www.sec.gov. The SEC allows us to incorporate by reference the information we file with them, which means that:

| • |     | incorporated documents are considered part of this prospectus supplement and the attached prospectus; |

| • |     | we can disclose important information to you by referring to those documents; and |

| • |     | information that we file with the SEC in the future and incorporate by reference herein will automatically update                                            
 and supersede information in this prospectus supplement and the attached prospectus and information previously incorporated by reference herein and therein. |

The information that we incorporate by reference is an important part of this prospectus supplement and the attached prospectus. Each document incorporated by reference is current only as of the date of such document, and the incorporation by reference of such documents shall not create any implication that there has been no change in our affairs since the date thereof or that the information contained therein is current as of any time subsequent to its date. Any statement contained in such incorporated documents shall be deemed to be modified or superseded for the purpose of this prospectus supplement and the attached prospectus to the extent that a subsequent statement contained in another document we incorporate by reference at a later date modifies or supersedes that statement. Any such statement so modified or superseded shall not be deemed, except as so modified or superseded, to constitute a part of this prospectus. We incorporate herein by reference:

| • |     | Our Annual Report on Form