<PAGE>

                                                                      EXHIBIT 31

                    CERTIFICATION OF CHIEF EXECUTIVE OFFICER
           PURSUANT TO SECTION 302 OF THE SARBANES-OXLEY ACT OF 2002

I, Charles J. Casamento, certify that:

1.    I have reviewed this quarterly report on Form 10-Q of Questcor
Pharmaceuticals, Inc.;

2.    Based on my knowledge, this report does not contain any untrue statement
of a material fact or omit to state a material fact necessary to make the
statements made, in light of the circumstances under which such statements were
made, not misleading with respect to the period covered by this report;

3.    Based on my knowledge, the financial statements, and other financial
information included in this report, fairly present in all material respects the
financial condition, results of operations and cash flows of the registrant as
of, and for, the periods presented in this report;

4.    The registrant's other certifying officer and I are responsible for
establishing and maintaining disclosure controls and procedures (as defined in
Exchange Act Rules 13a-15(e) and 15d-15(e)) for the registrant and have:

      a)    designed such disclosure controls and procedures, or caused such
disclosure controls and procedures to be designed under our supervision, to
ensure that material information relating to the registrant, including its
consolidated subsidiaries, is made known to us by others within those entities,
particularly during the period in which this report is being prepared;

      b)    evaluated the effectiveness of the registrant's disclosure controls
and procedures and presented in this report our conclusions about the
effectiveness of the disclosure controls and procedures, as of the end of the
period covered by this report based on such evaluation; and

      c)    disclosed in this report any change in the registrant's internal
control over financial reporting that occurred during the registrant's most
recent fiscal quarter that has materially affected, or is reasonably likely to
materially affect, the registrant's internal control over financial reporting;
and

5.    The registrant's other certifying officer and I have disclosed, based on
our most recent evaluation of internal control over financial reporting, to the
registrant's auditors and the audit committee of the registrant's board of
directors (or persons performing the equivalent functions):

      a)    all significant deficiencies and material weaknesses in the design
or operation of internal control over financial reporting which are reasonably
likely to adversely affect the registrant's ability to record, process,
summarize and report financial information; and

      b)    any fraud, whether or not material, that involves management or
other employees who have a significant role in the registrant's internal control
over financial reporting.

Date: May 13, 2004

                                      /s/ Charles J. Casamento
                                      ------------------------------
                                          CHARLES J. CASAMENTO
                                      CHAIRMAN, PRESIDENT AND CHIEF
                                            EXECUTIVE OFFICER

<PAGE>

                    CERTIFICATION OF CHIEF FINANCIAL OFFICER
            PURSUANT TO SECTION 302 OF THE SARBANES-OXLEY ACT OF 2002

I, Timothy E. Morris, certify that:

1.    I have reviewed this quarterly report on Form 10-Q of Questcor
Pharmaceuticals, Inc.;

2.    Based on my knowledge, this report does not contain any untrue statement
of a material fact or omit to state a material fact necessary to make the
statements made, in light of the circumstances under which such statements were
made, not misleading with respect to the period covered by this report;

3.    Based on my knowledge, the financial statements, and other financial
information included in this report, fairly present in all material respects the
financial condition, results of operations and cash flows of the registrant as
of, and for, the periods presented in this report;

4.    The registrant's other certifying officer and I are responsible for
establishing and maintaining disclosure controls and procedures (as defined in
Exchange Act Rules 13a-15(e) and 15d-15(e)) for the registrant and have:

      a)    designed such disclosure controls and procedures, or caused such
      disclosure controls and procedures to be designed under our supervision,
      to ensure that material information relating to the registrant, including
      its consolidated subsidiaries, is made known to us by others within those
      entities, particularly during the period in which this report is being
      prepared;

      b)    evaluated the effectiveness of the registrant's disclosure controls
      and procedures and presented in this report our conclusions about the
      effectiveness of the disclosure controls and procedures, as of the end of
      the period covered by this report based on such evaluation; and

      c)    disclosed in this report any change in the registrant's internal
      control over financial reporting that occurred during the registrant's
      most recent fiscal quarter that has materially affected, or is reasonably
      likely to materially affect, the registrant's internal control over
      financial reporting; and

5.    The registrant's other certifying officer and I have disclosed, based on
our most recent evaluation of internal control over financial reporting, to the
registrant's auditors and the audit committee of the registrant's board of
directors (or persons performing the equivalent functions):

      a)    all significant deficiencies and material weaknesses in the design
      or operation of internal control over financial reporting which are
      reasonably likely to adversely affect the registrant's ability to record,
      process, summarize and report financial information; and

      b)    any fraud, whether or not material, that involves management or
      other employees who have a significant role in the registrant's internal
      control over financial reporting.

Date: May 13, 2004

                                           /s/ Timothy E. Morris
                                           --------------------
                                              TIMOTHY E. MORRIS
                                           CHIEF FINANCIAL OFFICER