As filed with the Securities and Exchange Commission on December 31, 2014

Registration No. 333-175649

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM S-3

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

Post-Effective Amendment No. 1 to Form S-3 Registration Statement No. 333-175649

Walgreen Co.

(Exact name of registrant as specified in its charter)

108 Wilmot Road

Deerfield, Illinois 60015

(Address and Zip Code of Principal Executive Offices)

Thomas J. Sabatino, Jr.

Executive Vice President, Chief Legal and Administrative Officer and Corporate Secretary

108 Wilmot Road

Deerfield, Illinois 60015

Telephone: (847) 315-2500

(Name, Address, and Telephone Number, including Area Code, of Agent for Service)

Approximate date of commencement of proposed sale to the public: Not applicable.

If the only securities being registered on this Form are being offered pursuant to dividend or interest reinvestment plans, please check the following box.  ¨

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, other than securities offered only in connection with dividend or interest reinvestment plans, check the following box.  ¨

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ¨

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ¨

If this Form is a registration statement pursuant to General Instruction I.D. or a post-effective amendment thereto that shall become effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act, check the following box.  x

If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box.  ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act. (Check one):

EXPLANATORY NOTE

This Post-Effective Amendment No. 1 relates to the following Registration Statement on Form S-3 (the “Registration Statement”) of Walgreen Co. (the “Registrant”):

File No. 333-175649, pertaining to the registration of an indeterminate principal amount of debt securities.

On October 17, 2014, the Registrant entered into an Agreement and Plan of Merger (the “Reorganization Merger Agreement”) by and among the Registrant, Walgreens Boots Alliance, Inc. (“WBA”), a Delaware corporation and wholly owned subsidiary of the Registrant, and Ontario Merger Sub, Inc. (“Merger Sub”), a wholly owned subsidiary of WBA. Pursuant to the Reorganization Merger Agreement, on December 31, 2014, Merger Sub merged with and into the Registrant, with the Registrant continuing as the surviving corporation and becoming a wholly owned subsidiary of WBA (the “Reorg Merger”).

In connection with the consummation of the Reorg Merger, the offering pursuant to the Registration Statement has been terminated. In accordance with undertakings made by the Registrant in the Registration Statement to remove from registration, by means of a post-effective amendment, any of the securities which remain unsold at the termination of the offering, the Registrant hereby removes from registration the securities of the Registrant registered but remaining unsold under the Registration Statement. The Registration Statement is hereby amended, as appropriate, to reflect the deregistration of such securities.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, the Registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-3 and has duly caused this Post-Effective Amendment to be signed on its behalf by the undersigned, thereunto duly authorized, in the Village of Deerfield, State of Illinois, on December 31, 2014. No other person is required to sign this Post-Effective Amendment in reliance upon Rule 478 under the Securities Act of 1933, as amended.