Supplemental Prospectus (Form 424B3)

Filed pursuant to Rule 424(b)(3)
Registration No. 333-294900

PROSPECTUS SUPPLEMENT

(To the Prospectus Dated April 14, 2026)

This prospectus supplement updates, amends and supplements the prospectus dated April 14, 2026 (the "Prospectus"), which forms a part of our Registration Statement on Form S-1 (Registration No. 333-294900). Capitalized terms used in this prospectus supplement and not otherwise defined herein have the meanings specified in the prospectus.

This prospectus supplement is being filed to update, amend and supplement the information included in the Prospectus with the information contained in our Current Report on Form 8-K, filed with the Securities and Exchange Commission on June 9, 2026, which is set forth below.

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 common stock is listed on The Nasdaq Global Market under the symbol "LBRX." On June 8, 2026, the last reported sale price of our common stock was $29.40 per share.

Investing in our securities involves a high degree of risk. You should review carefully the risks and uncertainties described in the section titled "Risk Factors" beginning on page 8 of the Prospectus, and under similar headings in any amendments or supplements to the Prospectus.

Neither the Securities and Exchange Commission nor any other state securities commission has approved or disapproved of these securities or passed upon the accuracy or adequacy of the Prospectus or this prospectus supplement. Any representation to the contrary is a criminal offense.

The date of this prospectus supplement is June 9, 2026

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): June 3, 2026

LB Pharmaceuticals Inc

(Exact name of registrant as specified in its charter)

Delaware		001-42831		81-1854347
(state or other jurisdiction of incorporation)		(Commission File Number)		(I.R.S. Employer Identification No.)

One Pennsylvania Plaza, Suite 1025 New York, NY		10119
(Address of principal executive offices)		(Zip Code)

Registrant's telephone number, including area code: (212) 605-0300

Not applicable

(Former name or former address, if changed since last report.)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

☐	Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

☐	Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

☐	Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

☐	Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Securities registered pursuant to Section 12(b) of the Act:

Title of each class		Trading Symbol		Name of each exchange on which registered
Common Stock, $0.0001 par value per share		LBRX		The Nasdaq Stock Market LLC

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company ☒

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ☐

Item 5.07. Submission of Matters to a Vote of Security Holders.

LB Pharmaceuticals Inc (the "Company") held its 2026 Annual Meeting of Stockholders (the "Annual Meeting") on June 3, 2026, at which a quorum was present. The final results for each of the proposals submitted to a vote of the Company's stockholders at the Annual Meeting are set forth below. These proposals are described in the Company's definitive proxy statement filed with the Securities and Exchange Commission on April 23, 2026.

Proposal 1 - Election of Directors

The Company's stockholders elected the three persons listed below as Class I directors, each to serve a three-year term until the Company's 2029 Annual Meeting of Stockholders and until their successors are duly elected and qualified, or until their earlier death, resignation or removal. The final voting results were as follows:

		For		Withheld		Broker Non-Votes
Robert A. Lenz, M.D., Ph.D.		20,059,577		8,004		3,216,636
Rebecca Luse		18,530,680		1,536,901		3,216,636
Ran Nussbaum		18,748,511		1,319,070		3,216,636

Proposal 2 - Ratification of the Appointment of Independent Registered Public Accounting Firm

The Company's stockholders ratified the appointment of BDO USA, P.C. as the Company's independent registered public accounting firm for the fiscal year ending December 31, 2026. The final voting results were as follows:

For		Against		Abstain		Broker Non-Votes
23,275,974		5,490		2,753		-

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

Dated: June 9, 2026