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                                                                    EXHIBIT 4.13

                               drkoop.com, Inc.
                           7000 N. Mopac, Suite 400
                              Austin, Texas 78739


                                August 22, 2000


To: The trade creditor of
drkoop.com, Inc. identified
on the signature page of this
letter agreement (the "Trade Creditor")

Ladies and Gentlemen:

          The purpose of this letter agreement is to confirm that we have agreed
that drkoop.com,Inc. (the "Company") will pay to you ("DoubleClick"), within two
business days following the final closing of the proposed preferred stock
financing described in the Confidential Private Placement Memorandum dated July
12, 2000 and the related cumulative supplement dated August 16, 2000 (the
"Permanent Financing"), the sum in cash identified on the signature page of this
letter agreement (the "Cash Payment") and securities of the Company as described
on Attachment A (the "Securities" and, together with the Cash Payment, the
   ------------
"Compromise Amount").  The Cash Payment shall consist of 50% of the Compromise
Amount and the securities shall consist of 50% of the Compromise Amount.
Attachments A and B describe the terms of and contain representations and
warranties with respect to the issuance of the securities and constitute a part
of this letter agreement.  The Compromise Amount will represent full and final
payment of all of the Company's obligations due to you as of the date of this
agreement.  The compromise of the amounts due to you made in this letter
agreement is irrevocable.

          In consideration for the Compromise Amount, you hereby voluntarily
release and forever discharge the Company and its subsidiaries, affiliates,
directors, officers, employees, stockholders, agents and representatives, and
each of their successors and assigns (the "Releasees") from, and covenant not to
sue or proceed against the Releasees on the basis of, any and all past or
present causes of action, suits, agreements, losses, damages, debts, accounts,
expenses, obligations, liabilities or other claims, known or unknown, against
the Releasees in respect of any and all (i) contractual obligations or
liabilities, including trade receivables (except with respect to Company's
indemnification obligations set forth in the DART Services Agreement for
Publishers dated 11/15/83, (ii) claims for repayment of indebtedness for
borrowed money and (iii) claims for repayment of amounts owed to you arising
under bankruptcy, reorganization, insolvency, moratorium, fraudulent conveyance
or other similar laws affecting creditors' rights and the relief of debtors
generally.
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          This letter agreement shall terminate and be of no further force or
effect in the event that the Permanent Financing has not closed and the Company
has not delivered the Compromise Amount to you by September 1, 2000.

          Please confirm that the foregoing correctly sets forth the agreement
between us by signing in the space provided below.  This letter agreement may be
executed in counterpart.

                                             Sincerely yours,

                                             drkoop.com, Inc.


                                             By: /s/ Susan M. Georgen-Saad
                                                 -----------------------------
                                             Name:  Susan M. Georgen-Saad
                                             Title: Acting CFO


The foregoing is hereby acknowledged and agreed to as
of the date first above written.

Name of Trade Creditor:             DoubleClick, Inc.


                                    By: /s/ Cory Douglas
                                        ---------------------------
                                    Name:   Cory Douglas
                                    Title:  VP Finance - Techsolutions

Cash Payment:                       $129,810.32

Securities:                         See Attachment A

Amount payable as of
the date of this letter agreement:  $259,620.63

                                       2
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                                 ATTACHMENT A

                         Description of the Securities

Form of Security:   Common Stock
----------------

Issue Price:        $0.80 per share.
-----------

Legends:            Each certificate representing the Securities will be
-------             endorsed with the following legend (in addition to any
                    legend required under applicable state securities laws):

                    THE SECURITIES REPRESENTED BY THIS CERTIFICATE HAVE BEEN
                    ACQUIRED FOR INVESTMENT AND HAVE NOT BEEN REGISTERED UNDER
                    THE SECURITIES ACT OF 1933 OR ANY STATE SECURITIES LAW. SUCH
                    SECURITIES MAY NOT BE SOLD OR TRANSFERRED IN THE ABSENCE OF
                    SUCH REGISTRATION UNLESS SUCH SALE OR TRANSFER IS EXEMPT
                    FROM THE REGISTRATION AND PROSPECTUS DELIVERY REQUIREMENT OF
                    SAID ACT.

Registration:       The Company will include the Securities in the first
------------        registration statement (other than a registration relating
                    to employee benefit plans or pursuant to Rule 145
                    promulgated under the Securities Act of 1933, as amended
                    (the "Securities Act")) filed by the Company with the
                    Securities and Exchange Commission following the date of
                    this Agreement in order to permit the resale of such
                    Securities into the public market. The Trade Creditor will
                    provide any information about itself necessary to facilitate
                    the registration and will deliver a prospectus in connection
                    with any sale and otherwise abide by reasonable procedures
                    as established by the Company imposed upon selling
                    stockholders generally or as otherwise required to comply
                    with applicable law. Other stockholders of the Company will
                    also be permitted to participate in this registration.


                                      A-1
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                                 ATTACHMENT B

                        Representations and Warranties


     A.   Representations and Warranties of the Trade Creditor.  In connection
          ----------------------------------------------------
with the transfer of the Securities, the Trade Creditor represents to the
Company the following:

          1.   Experience. The Trade Creditor is an "accredited investor" as
               ----------
defined in Regulation D under the Securities Act.  The Trade Creditor is also
knowledgeable, sophisticated and experienced in making, and is qualified to make
decisions with respect to investments in securities such as this one, including
investments in securities issued by the Company and investments in comparable
companies, and has the ability to bear the economic risks of the investment.

          2.   Investment. The Trade Creditor is acquiring the Securities for
               ----------
investment for such Trade Creditor's own account and not with the view to, or
for resale in connection with, any distribution thereof.  The Trade Creditor
understands that the Securities have not been registered under the Securities
Act of 1933 by reason of a specific exemption from the registration provisions
of the Securities Act which depends upon, among other things, the bona fide
nature of the investment intent as expressed herein.  The Trade Creditor further
represents that he, she or it does not have any contract, undertaking, agreement
or arrangement with any person to sell, transfer or grant participation to any
third person with respect to any of the Securities.  The Trade Creditor
understands and acknowledges that the sale of the Securities by the Company to
the Trade Creditor will not be registered under the Securities Act on the ground
that this sale and the issuance of securities hereunder is exempt from the
registration requirements of the Securities Act.

          3.   Rule 144. The Trade Creditor acknowledges that the Securities
               --------
must be held indefinitely unless subsequently registered under the Securities
Act or an exemption from such registration is available.  The Trade Creditor is
aware of the provisions of Rule 144 promulgated under the Securities Act which
permit limited resale of securities purchased in a private placement subject to
the satisfaction of certain conditions, including, in case the Trade Creditor
has held the securities for less than two years or is an affiliate of the
Company, among other things: the availability of certain current public
information about the Company, the resale occurring not less than one year after
a party has purchased and paid for the securities to be sold, the sale being
through a "broker's transaction" or in transactions directly with a "market
maker," and the number of shares being sold during any three-month period not
exceeding specified limitations.  The Trade Creditor covenants that, in the
absence of an effective registration statement covering the securities in
question, the Trade Creditor will sell, transfer, or otherwise dispose of the
Securities only in a manner consistent with such Trade Creditor's
representations and covenants set forth herein.

          4.   Access to Information. The Trade Creditor has received and
               ---------------------
reviewed the Company's Confidential Private Placement Memorandum, dated as of
July 12, 2000, as

                                      B-1
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supplemented by the Cumulative Supplement, dated as of August 16, 2000
(including, without limitation, the section captioned "Risk Factors"), and has
had an opportunity to discuss the Company's business, management and financial
affairs with its management. The Trade Creditor understands that any such
discussions, as well as any written information issued by the Company, were
intended to describe the aspects of the Company's business and prospects which
the Company believes to be material, but were not necessarily a thorough or
exhaustive description.

     B.   Representations and Warranties of the Company.
          ---------------------------------------------

          1.   Organization.  The Company is duly organized and validly existing
               ------------
in good standing under the laws of the state of Texas. The Company has full
corporate power and corporate authority to own, operate and occupy its
properties and to conduct its business as presently conducted. The Company is
registered or qualified to do business and in good standing in each jurisdiction
in which it owns or leases property or transacts business and where the failure
to be so qualified would have a material adverse effect upon the financial
condition of the Company.

          2.   Due Authorization;  Legal, Valid and Binding Agreement.  The
               ------------------------------------------------------
Company has all requisite legal and corporate power and authority to execute,
deliver and perform its obligations under this agreement. To the extent any
shares of common stock are being issued to the Trade Creditor, such shares shall
be duly authorized, validly issued, fully paid and nonassessable. This agreement
has been duly authorized and validly executed and delivered by the Company and
constitutes a legal, valid and binding agreement of the Company enforceable
against the Company in accordance with its terms, except as enforceability may
be limited by applicable bankruptcy, insolvency, reorganization, moratorium or
similar laws affecting creditors' and contracting parties' rights generally and
except as enforceability may be subject to general principles of equity
(regardless of whether such enforceability is considered in a proceeding in
equity or at law).

                                      B-2