SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE TO

Tender Offer Statement under Section 14(d)(1) or 13(e)(1)

of the Securities Exchange Act of 1934

(Amendment No. 1)

AUXILIUM PHARMACEUTICALS, INC.

(Name of Subject Company (Issuer))

AUXILIUM PHARMACEUTICALS, INC.

(Names of Filing Person (Issuer))

1.50% Convertible Senior Notes Due 2018

(Title of Class of Securities)

05334D AA5

(CUSIP Number of Class of Securities)

Rajiv De Silva

President and Chief Executive Officer

Auxilium Pharmaceuticals, Inc.

640 Lee Road

Chesterbrook, Pennsylvania 19087

(484) 321-5901

(Name, address and telephone number of person authorized to receive notices and communications on behalf of the filing person)

With a copy to:

Alison S. Ressler

Sullivan & Cromwell LLP

1888 Century Park East, Suite 2100

Los Angeles, California 90067

(310) 712-6600

CALCULATION OF FILING FEE

Check the appropriate boxes below to designate any transactions to which the statement relates:

Check the following box if the filing is a final amendment reporting the results of the tender offer:  ¨

This Amendment No. 1 amends and supplements the Tender Offer Statement on Schedule TO filed with the Securities and Exchange Commission (the “SEC”) on February 3, 2015, (together with any amendments and supplements thereto, the “Schedule TO”) by Auxilium Pharmaceuticals, Inc., a Delaware corporation (the “Company”), which relates to the Company’s requirement to repurchase, at the option of holders of the Company’s 1.50% Convertible Senior Notes due 2018 (the “Notes”), 100% of the principal amount of the Notes, plus accrued and unpaid interest thereon to, but excluding March 5, 2015, pursuant to the terms and conditions of the Fundamental Change Purchase Right Notice, Notice of Right to Convert and Notice of Entry into Supplemental Indenture and Offer to Purchase dated February 3, 2015 (the “Notice”).

This Schedule TO is intended to satisfy the disclosure requirements of Rules 13e-4(c)(2) and 13e-4(d)(1) under the Securities Exchange Act of 1934, as amended. All capitalized terms used but not specifically defined in this Schedule TO have the meanings given to such terms in the Notice.

Items 1 through 9, and Item 11.

Items 1 through 9, and Item 11 of the Schedule TO are hereby amended and supplemented by replacing each instance of the phrase “surrendered for conversion and not validly withdrawn” set forth in the Notice with the phrase “surrendered for conversion.”

The amendment and supplement to the Notice is attached hereto as Exhibit (a)(1)(B) and incorporated herein by reference.

Item 12. Exhibits.

Item 12 of the Schedule TO is hereby amended and supplemented to include the following exhibit:

SIGNATURES

After due inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Date: February 4, 2015

EXHIBIT INDEX

Exhibit (a)(1)(B)

AMENDMENT AND SUPPLEMENT TO

FUNDAMENTAL CHANGE PURCHASE RIGHT NOTICE, NOTICE OF RIGHT TO

CONVERT AND NOTICE OF ENTRY INTO SUPPLEMENTAL INDENTURE AND

OFFER TO PURCHASE TO HOLDERS OF 1.50% CONVERTIBLE SENIOR

NOTES DUE 2018

ISSUED BY

AUXILIUM PHARMACEUTICALS, INC.

CUSIP NUMBER: 05334D AA5

This Amendment and Supplement to the Fundamental Change Purchase Right Notice, Notice of Right to Convert and Notice of Entry into Supplemental Indenture and Offer to Purchase, dated February 3, 2015, to Holders of 1.50% Convertible Senior Notes due 2018 (the “Notice”) is being made on February 4, 2015 pursuant to the terms and conditions of the Indenture, dated as of January 30, 2013 (the “Base Indenture”), between Auxilium Pharmaceuticals, Inc., a Delaware corporation (the “Company”), and Wells Fargo Bank, National Association, a national banking association, as trustee (the “Trustee”), as supplemented by the First Supplemental Indenture, dated as of January 30, 2013 (the “First Supplemental Indenture”), between the Company and the Trustee and the Second Supplemental Indenture, dated as of January 29, 2015, among the Company, Endo International plc, a public limited company incorporated under the laws of Ireland, and the Trustee (the “Second Supplemental Indenture,” and the Base Indenture together with the First Supplemental Indenture and the Second Supplemental Indenture, the “Indenture”), relating to the Company’s 1.50% Convertible Senior Notes due 2018 (the “Notes”).

This Amendment and Supplement modifies and should be read in conjunction with the Notice.

The Paying Agent and Conversion Agent is:

Wells Fargo Bank, National Association

Or

By Facsimile Transmission:

(For Eligible Institutions only):

Fax: (612) 667-6282

Attn: Bondholder Communications

Phone Number:

(800) 344-5128

The following material amends and supplements, but does not materially change, the Notice to the extent set forth herein, and should be read in conjunction with the Notice.

The Notice is hereby amended and supplemented by replacing each instance of the phrase “surrendered for conversion and not validly withdrawn” with the phrase “surrendered for conversion.”