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EXHIBIT 10.43 [QUESTCOR LETTERHEAD] February 3, 2004 VIA EMAIL
Reinhard Koenig, M.D., Ph.D.
3260 Whipple Road
Union City, CA 94587 Re: Offer of Employment Dear Dr. Koenig: Questcor Pharmaceuticals, Inc. (the Company) is pleased to offer you the position of Vice
President, Medical Affairs, on the terms described below. Should you accept our offer of
employment, your start date will be on or before February 8, 2004. You will report to R. Jerald Beers, Vice President, Sales and Marketing. Your office will be
located at our facility in Union City, California. Of course, the Company may change your
reporting responsibilities, position, duties, and work location from time to time, as it deems
necessary. Your base compensation will be $170,000 per annum ($7,083.33 semi-monthly) less all amounts the
Company is required to hold under applicable laws. During your first two weeks of employment, the
Company will extend to you a note in the amount of $50,000, the terms of which will be covered in
the Promissory Note. You will be a participant in the annual management incentive program for
executives, which has been approved by the Compensation Committee. Your incentive bonus will be
based on the attainment of specific milestones during each calendar year. Said milestones will be
communicated to you in writing by Mr. Beers following the start of your employment and will be
updated annually as part of the performance review process. Your bonus opportunity will be 33% of
your base compensation earnings in the calendar year to which it applies. For 2004, 50% of your
bonus opportunity will be based on the attainment of mandatory corporate objectives, which have yet
to be determined by the board of directors. The bonus opportunity will not become effective
unless or until the corporate objectives have been met. You will be eligible to participate in the Companys various benefit plans including medical,
dental and vision insurance, as well as life, accidental death and disability insurance. You will
receive 16 days of paid vacation per calendar year, in addition to 12 paid regular holidays and two
paid floating holidays. You will also be eligible to




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participate in the Companys 401(k) Plan, Section 529 College Savings Program and
Employee Stock Purchase Plan. The eligibility requirements for these plans are explained in the
Companys Employee Handbook, and in the case of the Companys 401(k) Plan, in the 401(k) Plans
summary plan description. A copy of the Employee Handbook and the 401(k) Plans summary plan
description will be provided to you. Please read them carefully. Of course, to the extent the
provisions of the various plans are inconsistent with the provisions of the Employee Handbook or
summary plan description, the plan provisions will control. As you no doubt appreciate, as a Company employee, you will be expected to abide by Company rules
and regulations, acknowledge in writing that you have read the Companys Employee Handbook, sign
and comply with a Proprietary Information and Inventions Agreement which prohibits unauthorized use
or disclosure of Company proprietary information and sign the Policy Against Insider Trading. The Companys management has in effect an employee stock option plan to recognize the talent and
skills our employees bring to the Company. Management will recommend to the Board of Directors
that, at the time you join the Company, the Company grant to you an option under the stock option
plan to purchase 250,000 shares of the Common Stock of the Company at an exercise price equal to
100% of the closing price of the Companys Common Stock on the date prior to approval by the Board
of Directors. One-eighth (1/8th) of these options will vest after six (6) months of
employment and thereafter the remaining shares will vest at the rate of 1/48th of the
total grant on each monthly anniversary of your continued employment with the Company. The option
will be subject to the terms and conditions of the Companys stock option plan and your stock
option agreement. The Company will review your performance in accordance with the Employee Handbook, to assess your
accomplishment of milestones and goals, which the Company reasonably sets for you. The Company will
consider whether and when you should receive increases in your compensation and benefits as
described therein based on such accomplishments. You may terminate your employment with the Company at any time and for any reason whatsoever simply
by notifying the Company. Likewise, the Company may terminate your employment at any time and for
any reason whatsoever, with or without cause or advance notice. This at-will employment
relationship cannot be changed except in writing signed by the Chief Executive Officer or the Chief
Financial Officer. Any and all disputes connected with, relating to or arising from your employment with the Company
will be settled by final and binding arbitration in accordance with the rules of the American
Arbitration Association as presently in force. The only claims not covered by this Agreement are
claims for benefits under the unemployment insurance or workers compensation laws. Any such
arbitration will take place in Alameda County, California. The parties hereby incorporate into
this agreement all of the arbitration provisions of Section 1283.05 of the California Code of Civil
Procedure. The Company understands and agrees that it will bear the costs of the arbitration
filing and hearing fees and the cost of the arbitrator. Each side will bear its own attorneys
fees, and the arbitrator will not have authority to award 2.




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attorneys fees unless a statutory section at issue in the dispute authorizes the award of attorneys fees to the prevailing party, in
which case the arbitrator has authority to make such
award as permitted by the statute in question. The arbitration shall be instead of any civil
litigation; this means that you are waiving any right to a jury trial, and that the arbitrators
decision shall be final and binding to the fullest extent permitted by law and enforceable by any
court having jurisdiction thereof. Judgment upon any award rendered by the arbitrators may be
entered in any court having jurisdiction. The employment terms in this letter supersede any other agreements or promises made to you by
anyone, whether oral or written, express or implied. In the event you accept this employment
offer, the terms set forth in this letter will comprise our final, complete and exclusive agreement
with respect to the subject matter of this letter. Thus, by accepting this employment offer and
signing this offer letter, you agree to be bound by its terms and conditions. As required by law,
the Companys offer is subject to satisfactory proof of your right to work in the United States no
later than three days after your employment begins. Please sign and date this letter, and return it to me as soon as possible. This offer terminates if
it is not signed and delivered to me by February 5, 2004. A facsimile copy will suffice for this
purpose, so long as an original signature is delivered when you commence employment. My
confidential facsimile number is (928) 244-3718. We look forward to your favorable reply and to a productive and enjoyable work relationship. Sincerely, /s/ FREDRIC I. STORCH
 Fredric I. Storch
Senior Director, Human Resources and Administration I hereby acknowledge that I have read the foregoing letter and agree to be bound by all of its
terms and conditions: /s/ REINHARD KOENIGReinhard Koenig, M.D., Ph.D.February 4, 2004Date
 3.