SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549
--------------------------------------------------------------------------------

                                  SCHEDULE 13D
                    Under the Securities Exchange Act of 1934

                             Avanir Pharmaceuticals
                             ----------------------
                                (Name of Issuer)

                                  Common Stock
                         ------------------------------
                         (Title of Class of Securities)

                                   05348P104
                                -----------------
                                 (CUSIP Number)

                                 Samuel D. Isaly
                              OrbiMed Advisors LLC
                             OrbiMed Capital II LLC
                                767 Third Avenue
                               New York, NY 10017
                            Telephone: (212) 739-6400
                   ------------------------------------------
                     (Name, Address and Telephone Number of
                    Person Authorized to Receive Notices and
                                 Communications)

                                    Copy to:

                             Paul S. Schreiber, Esq.
                             Shearman & Sterling LLP
                              599 Lexington Avenue
                            New York, New York 10022
                            Telephone: (212) 848-4000

                               September 16, 2004
                               ------------------
             (Date of Event which Requires Filing of this Statement)

--------------------------------------------------------------------------------

If the filing person has previously filed a statement on Schedule 13G to report
the acquisition which is the subject of this Schedule 13D, and is filing this
schedule because of Rule 13d-1(e), 13d-1(f), or 13d-1(g), check the following
box |_|.

Check the following box if a fee is being paid with this Statement |_|.



                                  Page 1 of 14



CUSIP No. 068750V102


(1)     Name of Reporting Person
        S.S. or I.R.S. Identification No. of Above Person

        Samuel D. Isaly
        -----------------------------------------------------------------------
        -----------------------------------------------------------------------

(2)     Check the Appropriate Box if a Member of a Group (See Instructions)

|_|     (a)
             ------------------------------------------------------------------

|_|     (b)
             ------------------------------------------------------------------
             ------------------------------------------------------------------

(3)     SEC Use Only
                     ----------------------------------------------------------
        -----------------------------------------------------------------------

(4)     Source of Funds (See Instructions) AF
                                          -------------------------------------
        -----------------------------------------------------------------------

(5)     Check if Disclosure of Legal Proceedings is Required Pursuant to Item
        2(d) or 2(e).
|_|
        -----------------------------------------------------------------------

(6)     Citizenship or Place of Organization  United States
                                             ----------------------------------
        -----------------------------------------------------------------------

----------------
                     (7)   Sole Voting Power
   Number of                                 -------------------------------
    Shares                 -------------------------------------------------
 Beneficially        (8)   Shared Voting Power   7,875,000
   Owned by                                    -----------------------------
     Each            (9)   Sole Dispositive Power
   Reporting                                      --------------------------
  Person With              -------------------------------------------------
                    (10)   Shared Dispositive Power   7,875,000
                                                    ------------------------
----------------           -------------------------------------------------

(11)    Aggregate Amount Beneficially Owned by Each Reporting Person  7,875,000
                                                                      ---------
        -----------------------------------------------------------------------

(12)    Check if the Aggregate Amount in Row (11) Excludes Certain Shares
        (See Instructions)
 |_|
        -----------------------------------------------------------------------


(13)    Percent of Class Represented by Amount in Row (11)   8.63%
                                                            -------------------
        -----------------------------------------------------------------------

(14)    Type of Reporting Person (See Instructions)  IN
                                                    ---------------------------
        -----------------------------------------------------------------------



                                  Page 2 of 14



CUSIP No. 068750V102


(1)     Name of Reporting Person
        S.S. or I.R.S. Identification No. of Above Person

        OrbiMed Advisors LLC
        -----------------------------------------------------------------------
        -----------------------------------------------------------------------

(2)     Check the Appropriate Box if a Member of a Group (See Instructions)

|_|     (a)
             ------------------------------------------------------------------

|_|     (b)
             ------------------------------------------------------------------
             ------------------------------------------------------------------

(3)     SEC Use Only
                     ----------------------------------------------------------
        -----------------------------------------------------------------------

(4)     Source of Funds (See Instructions) AF
                                          -------------------------------------
        -----------------------------------------------------------------------

(5)     Check if Disclosure of Legal Proceedings is Required Pursuant to Item
        2(d) or 2(e).
|_|
        -----------------------------------------------------------------------

(6)     Citizenship or Place of Organization  Delaware
                                             ----------------------------------
        -----------------------------------------------------------------------

----------------
                     (7)   Sole Voting Power
   Number of                                 -------------------------------
    Shares                 -------------------------------------------------
 Beneficially        (8)   Shared Voting Power   651,473
   Owned by                                    -----------------------------
     Each            (9)   Sole Dispositive Power
   Reporting                                      --------------------------
  Person With              -------------------------------------------------
                    (10)   Shared Dispositive Power   651,473
                                                    ------------------------
----------------           -------------------------------------------------

(11)    Aggregate Amount Beneficially Owned by Each Reporting Person   651,473
                                                                      ---------
        -----------------------------------------------------------------------

(12)    Check if the Aggregate Amount in Row (11) Excludes Certain Shares
        (See Instructions)
 |_|
        -----------------------------------------------------------------------


(13)    Percent of Class Represented by Amount in Row (11)   0.71%
                                                            -------------------
        -----------------------------------------------------------------------

(14)    Type of Reporting Person (See Instructions)  IA
                                                    ---------------------------
        -----------------------------------------------------------------------



                                  Page 3 of 14



CUSIP No. 068750V102


(1)     Name of Reporting Person
        S.S. or I.R.S. Identification No. of Above Person

        OrbiMed Capital II LLC
        -----------------------------------------------------------------------
        -----------------------------------------------------------------------

(2)     Check the Appropriate Box if a Member of a Group (See Instructions)

|_|     (a)
             ------------------------------------------------------------------

|_|     (b)
             ------------------------------------------------------------------
             ------------------------------------------------------------------

(3)     SEC Use Only
                     ----------------------------------------------------------
        -----------------------------------------------------------------------

(4)     Source of Funds (See Instructions) AF
                                          -------------------------------------
        -----------------------------------------------------------------------

(5)     Check if Disclosure of Legal Proceedings is Required Pursuant to Item
        2(d) or 2(e).
|_|
        -----------------------------------------------------------------------

(6)     Citizenship or Place of Organization  Delaware
                                             ----------------------------------
        -----------------------------------------------------------------------

----------------
                     (7)   Sole Voting Power
   Number of                                 -------------------------------
    Shares                 -------------------------------------------------
 Beneficially        (8)   Shared Voting Power   7,223,527
   Owned by                                    -----------------------------
     Each            (9)   Sole Dispositive Power
   Reporting                                      --------------------------
  Person With              -------------------------------------------------
                    (10)   Shared Dispositive Power   7,223,527
                                                    ------------------------
----------------           -------------------------------------------------

(11)    Aggregate Amount Beneficially Owned by Each Reporting Person  7,223,527
                                                                      ---------
        -----------------------------------------------------------------------

(12)    Check if the Aggregate Amount in Row (11) Excludes Certain Shares
        (See Instructions)
 |_|
        -----------------------------------------------------------------------


(13)    Percent of Class Represented by Amount in Row (11)   7.92%
                                                            -------------------
        -----------------------------------------------------------------------

(14)    Type of Reporting Person (See Instructions)  CO
                                                    ---------------------------
        -----------------------------------------------------------------------



                                  Page 4 of 14



Item 1.  Security and Issuer.
         -------------------

The class of equity securities to which this Statement on Schedule 13D relates
is the Common Stock (the "Shares"), of Avanir Pharmaceuticals (the "Issuer"), a
corporation with its principal executive offices located at 11388 Sorrento
Valley Road, Suite 200, San Diego, 92121.

Item 2.  Identity and Background.
         -----------------------

         (a) This statement is being filed by Samuel D. Isaly, an individual
("Isaly"), and by OrbiMed Advisors LLC and OrbiMed Capital II LLC, limited
liability companies organized under the laws of Delaware.

         (b)-(c) OrbiMed Advisors LLC is a registered investment adviser under
the Investment Advisers Act of 1940, as amended that acts as investment adviser
or general partner to certain clients which hold Shares of the Issuer, as more
particularly described in Item 6 below. OrbiMed Advisors LLC has its principal
offices at 767 Third Avenue, 30th Floor, New York, NY 10017.

         OrbiMed Capital II LLC is a registered investment advisor under the
Investment Advisers Act of 1940, as amended that acts as investment adviser or
general partner to certain limited partnerships as more particularly described
in Item 6 below. OrbiMed Capital II LLC has its principal offices at 767 Third
Avenue, 30th Floor, New York, NY 10017.

         Isaly, a natural person, owns a controlling interest in OrbiMed
Advisors LLC and OrbiMed Capital II LLC.

         The directors and executive officers of OrbiMed Advisors LLC and
OrbiMed Capital II LLC are set forth on Schedules I and II, attached hereto.
Schedules I and II set forth the following information with respect to each such
person:

         (i) name;

         (ii) business address (or residence address where indicated);

         (iii) present principal occupation or employment and the name,
     principal business and address of any corporation or other organization
     in which such employment is conducted; and

         (iv) citizenship.

         (d)-(e) During the last five years, neither the Reporting Persons nor
any Person named in Schedules I through IV have been (i) convicted in a criminal
proceeding (excluding traffic violations or similar misdemeanors) or (ii) a
party to a civil proceeding of a judicial or administrative body of competent
jurisdiction and as a result of such proceeding was or is subject to a judgment,
decree or final order enjoining future violations of, or prohibiting or
mandating activities subject to, federal or state securities laws or finding any
violation with respect to such laws.



                                  Page 5 of 14



         (f) Isaly is a citizen of the United States.

Item 3.  Source and Amount of Funds or Other Consideration.
         -------------------------------------------------

         Prior to May 26, 2004, pursuant to the authority of OrbiMed Advisors
LLC, OrbiMed Advisors Inc. and OrbiMed Capital II LLC under their respective
investment advisory contracts and limited partnership agreements with or
relating to Caduceus Private Investments II, LP, Caduceus Private Investments II
(QP), LP and UBS Juniper Crossover Fund LLC as more particularly referred to in
Item 6 below caused these clients to purchase 7,875,000 Shares of the Issuer.

         Following the above transactions, and as a result of their common
control and mutual affiliation, the Reporting Persons were the beneficial owners
of approximately 8.63% of the outstanding Shares of the Issuer.

         None of the Reporting Persons have acquired or disposed of any
additional Shares of the Issuer since May 26, 2004.

Item 4.  Purpose of Transaction.
         ----------------------

         As described more fully in Item 3 above, this statement relates to the
distribution and disposal of Shares by the Reporting Persons. The Shares were
acquired, i.e., for the purpose of making an investment in the Issuer and not
with the intention of acquiring control of the Issuer's business on behalf of
the Reporting Persons respective advisory clients.

         The Reporting Persons were previously granted and exercised the right
to appoint a representative to the Board of Directors (the "Board") of the
Issuer, so as to permit active monitoring of the operations of the Issuer.
Jonathan T. Silverstein currently serves as the Reporting Persons'
representative to the Board.

         The Reporting Persons from time to time intend to review their
investment in the Issuer on the basis of various factors, including the Issuer's
business, financial condition, results of operations and prospects, general
economic and industry conditions, the securities markets in general and those
for the Issuer's Shares in particular, as well as other developments and other
investment opportunities. Based upon such review, the Reporting Persons will
take such actions in the future as the Reporting Persons may deem appropriate in
light of the circumstances existing from time to time. If the Reporting Persons
believe that further investment in the Issuer is attractive, whether because of
the market price of the Issuer's Shares or otherwise, they may acquire shares of
common stock or other securities of the Issuer either in the open market or in
privately negotiated transactions. Similarly, depending on market and other
factors, the Reporting Persons may determine to dispose of some or all of the
Shares currently owned by the Reporting Persons or otherwise acquired by the
Reporting Persons either in the open market or in privately negotiated
transactions.

         Except as set forth above, the Reporting Persons have not formulated
any plans or proposals which relate to or would result in: (a) the acquisition
by any person of additional securities of the Issuer or the disposition of
securities of the Issuer, (b) an extraordinary corporate transaction involving
the Issuer or any of its subsidiaries, (c) a sale or transfer of a material



                                  Page 6 of 14



amount of the assets of the Issuer or any of its subsidiaries, (d) any change in
the present board of directors or management of the Issuer, (e) any material
change in the Issuer's capitalization or dividend policy, (f) any other material
change in the Issuer's business or corporate structure, (g) any change in the
Issuer's charter or bylaws or other or instrument corresponding thereto or other
action which may impede the acquisition of control of the Issuer by any person,
(h) causing a class of the Issuer's securities to be deregistered or delisted,
(i) a class of equity securities of the Issuer becoming eligible for termination
of registration or (j) any action similar to any of those enumerated above.

Item 5.  Interest in Securities of the Issuer.
         ------------------------------------

         (a)-(b) As of this date of this filing, Samuel D. Isaly, OrbiMed
Advisors LLC and OrbiMed Capital II LLC may be deemed directly or indirectly,
including by reason of their mutual affiliation, to be the beneficial owners of
the Common Stock described in Item 3. Based upon information contained in the
most recent available filing by the Issuer with the SEC, such Shares constitute
approximately 8.63% of the issued and outstanding Shares. As described above in
Item 2, Isaly owns, pursuant to the terms of the limited liability company
agreement of each of OrbiMed Advisors LLC and OrbiMed Capital II LLC, a
controlling interest in the outstanding limited liability company interests of
such entity. As a result, Samuel D. Isaly, OrbiMed Advisors LLC and OrbiMed
Capital II LLC share power to direct the vote and to direct the disposition of
the Common Stock described in Item 3.

         (c) Except as disclosed in Item 3, the Reporting Persons have not
effected any transaction in the Shares during the past 60 days.

         (d) Not applicable.

         (e) Not applicable.

Item 6.  Contracts, Arrangements, Understandings or Relationships with Respect
         to Securities of the Issuer.
         ---------------------------------------------------------------------

In addition to the relationships between the Reporting Persons described in Item
5, OrbiMed Capital II LLC is the general partner of Caduceus Private Investments
II, LP ("Caduceus") and Caduceus Private Investments II (QP) ("Caduceus QP"),
LP, private equity funds, pursuant to the terms of their respective limited
partnership agreements. OrbiMed Advisors LLC, through a joint venture with UBS
Fund Advisor, LLC entitled PW Juniper Management, LLC, acts as investment
manager of UBS Juniper Crossover Fund LLC ("Juniper"), a registered investment
company, pursuant to the terms of the UBS Juniper Crossover Fund, LLC investment
advisory agreement. Pursuant to these agreements and relationships, OrbiMed
Advisors LLC and OrbiMed Capital II LLC have discretionary investment management
authority with respect to the assets of the above-listed investment accounts.
Such authority includes the power to vote and otherwise dispose of securities
purchased by Caduceus, Caduceus QP and Juniper including the total Shares of
Common Stock of the Issuer held by Caduceus, Associates, Juniper, Finsbury and
Eaton Vance. The number of outstanding Shares of the Issuer attributable to each
of these is 5,255,689, 1,967,838 and 651,473. As noted above under Item 4,
OrbiMed Advisors LLC has the authority



                                  Page 7 of 14



to appoint a representative to the Board of Directors of the Issuer and
accordingly the Reporting Persons may have the ability to effect and influence
control of the Issuer.

         Other than the agreements and the relationships mentioned above, to the
best knowledge of the Reporting Persons, there are no contracts, arrangements,
understandings or relationships (legal or otherwise) among the persons named in
Item 2 and between such persons and any persons with respect to any securities
of the Issuer, including, but not limited to, transfer or voting of any of the
Shares, finder's fees, joint ventures, loan or option arrangements, puts or
calls, guarantees of profits, division of profits or loss, or the giving of
withholding of proxies.



                                  Page 8 of 14



Item 7.  Material to Be Filed as Exhibits.
         --------------------------------

Exhibit                       Description
-------                       -----------

   A.  Joint Filing Agreement among Samuel D. Isaly, OrbiMed Advisors LLC and
       OrbiMed Capital II LLC.



                                  Page 9 of 14



Signature



                  After reasonable 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.

September 30, 2004                                  SAMUEL D. ISALY


                                                    By:   /s/ Samuel D. Isaly
                                                        ------------------------
                                                        Name:  Samuel D. Isaly


                                                    ORBIMED ADVISORS LLC


                                                    By:   /s/ Samuel D. Isaly
                                                        ------------------------
                                                        Name:  Samuel D. Isaly
                                                        Title:  Managing Member



                                                    ORBIMED CAPITAL II LLC


                                                    By:   /s/ Samuel D. Isaly
                                                        ------------------------
                                                        Name:  Samuel D. Isaly
                                                        Title:  Managing Member



                                 Page 10 of 14



                                   Schedule I

                  The name and present principal occupation of each of the
executive officers and directors of OrbiMed Advisors LLC are set forth below.
Unless otherwise noted, each of these persons are United States citizens and
have as their business address 767 Third Avenue, New York, NY 10017.



=========================================================================================
                                    Position with Reporting
            Name                            Person                  Principal Occupation
-----------------------------------------------------------------------------------------
                                                            
Samuel D. Isaly                   Managing Partner                Partner
                                                                  OrbiMed Advisors LLC
-----------------------------------------------------------------------------------------
Michael Sheffery                  Partner                         Partner
                                                                  OrbiMed Advisors LLC
-----------------------------------------------------------------------------------------
Carl L. Gordon                    Partner                         Partner
                                                                  OrbiMed Advisors LLC
-----------------------------------------------------------------------------------------
Sven Borho                        Partner                         Partner
   German and Swedish Citizen                                     OrbiMed Advisors LLC
-----------------------------------------------------------------------------------------
Jonathan T. Silverstein           Partner                         Partner
                                                                  OrbiMed Advisors LLC
-----------------------------------------------------------------------------------------
Eric A. Bittelman                 Chief Financial Officer         Chief Financial Officer
                                                                  OrbiMed Advisors LLC
=========================================================================================




                                 Page 11 of 14



                                   Schedule II


                  The name and present principal occupation of each of the
executive officers and directors of OrbiMed Capital II LLC are set forth below.
Unless otherwise noted, each of these persons are United States citizens and
have as their business address 767 Third Avenue, New York, NY 10017.




=========================================================================================
                                    Position with Reporting
            Name                            Person                  Principal Occupation
-----------------------------------------------------------------------------------------
                                                            
Samuel D. Isaly                   Managing Partner                Partner
                                                                  OrbiMed Capital II LLC
-----------------------------------------------------------------------------------------
Michael Sheffery                  Partner                         Partner
                                                                  OrbiMed Capital II LLC
-----------------------------------------------------------------------------------------
Carl L. Gordon                    Partner                         Partner
                                                                  OrbiMed Capital II LLC
-----------------------------------------------------------------------------------------
Sven Borho                        Partner                         Partner
   German and Swedish Citizen                                     OrbiMed Capital II LLC
-----------------------------------------------------------------------------------------
Jonathan T. Silverstein           Partner                         Partner
                                                                  OrbiMed Capital II LLC
-----------------------------------------------------------------------------------------
Eric A. Bittelman                 Chief Financial Officer         Chief Financial Officer
                                                                  OrbiMed Capital II LLC
=========================================================================================




                                 Page 12 of 14



                                  EXHIBIT INDEX


--------------------------------------------------------------------------------
    Exhibit                        Description                          Page No.
    -------                        -----------                          --------
--------------------------------------------------------------------------------
       A.        Joint Filing Agreement among Samuel D. Isaly,            A-1
                 OrbiMed Advisors LLC and OrbiMed Capital, LLC.
--------------------------------------------------------------------------------



                                 Page 13 of 14



                             JOINT FILING AGREEMENT

         The undersigned hereby agree that the Statement on Schedule 13D, dated
September 16, 2004, (the "Schedule 13D"), with respect to the Common Stock, no
par value per share, of Avanir Pharmaceuticals is, and any amendments thereto
executed by each of us shall be, filed on behalf of each of us pursuant to and
in accordance with the provisions of Rule 13d-1(f) under the Securities and
Exchange Act of 1934, as amended, and that this Agreement shall be included as
an Exhibit to the Schedule 13D and each such amendment. Each of the undersigned
agrees to be responsible for the timely filing of the Schedule 13D and any
amendments thereto, and for the completeness and accuracy of the information
concerning itself contained therein. This Agreement may be executed in any
number of counterparts, all of which taken together shall constitute one and the
same instrument.

         IN WITNESS WHEREOF, the undersigned have executed this Agreement as of
the 30th day of September, 2004.


SAMUEL D. ISALY                              ORBIMED ADVISORS LLC



 /s/ Samuel D. Isaly                          /s/ Samuel D. Isaly
-----------------------------------          -----------------------------------
                                             Name:  Samuel D. Isaly
                                             Title:  President



ORBIMED CAPITAL II LLC



 /s/ Samuel D. Isaly
-----------------------------------
Name:  Samuel D. Isaly
Title:  President



                                 Page 14 of 14