Exhibit 10.21

AMENDMENT NO. 1 TO STOCK PURCHASE AGREEMENT

This Amendment (this “Amendment”) to the Stock Purchase Agreement (“Stock Purchase Agreement”) entered into on May 31, 2011, by and among LecTec Corporation, a Minnesota corporation (the “Company”) and the Persons listed on Schedule A attached hereto, as such Schedule A may be amended from time to time (individually, a “Purchaser” and collectively, the “Purchasers”) is entered into on June       , 2011 (“Effective Date”).

WHEREAS, the Company and the Purchasers wish to amend the Stock Purchase Agreement under the terms and subject to the conditions set forth in this Amendment and the Stock Purchase Agreement as if set forth herein;

In consideration of the mutual promises contained herein, the parties agree as follows:

1.1 Section 1.1 of the Stock Purchase Agreement is hereby amended and restated in its entirety as set forth below:

“1.1 Sale and Purchase of Shares. On the terms and subject to the conditions set forth herein, at the Closing (as defined below), the Company shall issue and sell to each Purchaser, and each Purchaser shall purchase from the Company Shares of common stock for an aggregate purchase price of $1.0 million at the Investor Stock Purchase Price (as such term is defined in the Agreement and Plan of Merger by and among LecTec Corporation, Nerve Merger Sub Corp. and Company on May 31, 2011, as amended to date). Each Purchaser’s investment is set forth in Column A on the attached Schedule A, with the number of Shares to be that number of fully paid and nonassessable whole shares of common stock of the Company calculated based upon the investment amount per each Purchaser as set forth in Schedule A. The Company’s agreement with each of the Purchasers is a separate agreement and the sale of the Shares to each of the Purchasers is a separate sale.”

1.2 Schedule A of the Stock Purchase Agreement is hereby amended and restated in its entirety as attached.

2.1 Except as defined herein, all defined terms used herein shall have the meaning set forth in the Stock Purchase Agreement

2.2 The captions to the paragraphs/sections in this Amendment are not a part of this Amendment or the Stock Purchase Agreement, and are included merely for convenience of reference only and shall not affect its meaning or interpretation.

2.3 This Amendment may be signed in any number of counterparts with the same effect as if the signatures thereto and hereto were upon the same instrument.

2.4 This Amendment was drafted by all parties concerned and thus any rule of contract interpretation calling for documents to be construed against the drafter shall not apply to the construction of this Amendment.

2.5 Company and each of the Purchasers confirm and acknowledge that the Stock Purchase Agreement is in full force and effect, that there have been no uncured events of breach to date, and that each represents and warrants to the other that they are in material compliance with the Stock Purchase Agreement. Except for the changes made by this Amendment to the Stock Purchase Agreement, the Stock Purchase Agreement remains in full force and effect without modification. All references to the Agreement in the Stock Purchase Agreement mean the Stock Purchase Agreement as amended hereby.

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The parties hereto have executed this Amendment as of the day and year first set forth above.

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SCHEDULE A

SCHEDULE OF PURCHASERS