Document:

<PAGE>

                                                                   EXHIBIT 10.47

                              SECURITY AGREEMENT

          THIS SECURITY AGREEMENT (the "Agreement") is entered into by and among
HealthCentral.com, a Delaware corporation (the "Debtor"), and the persons and
organizations whose signatures appear below, as may be amended to include
additional Purchasers pursuant to the Purchase Agreement (each a "Secured Party"
                                                                  -------------
and collectively the "Secured Parties"), effective as of May 10, 2001.  For
                      ---------------
purposes of this Agreement, the term "Secured Parties" includes all subsequent
holders of Notes (as defined below).  Any capitalized terms used herein but not
defined herein shall have the meanings ascribed to them in that certain Note and
Warrant Purchase Agreement, dated May 10, 2001 by and among the Debtor and the
Secured Parties (the "Purchase Agreement").
                      ------------------

          In consideration of the mutual promises contained herein, and as an
inducement to the Debtor to repay to the Secured Parties the amounts reflected
by the Notes (as defined below), the parties agree as follows:

          1.  Creation of a Security Interest.  As security for payment of the
              -------------------------------
Indebtedness (as defined below) of Debtor to the Secured Parties when and as
due, Debtor hereby grants the Secured Parties a security interest in the
collateral described in Section 2 below (the "Collateral").  For purposes of
this Agreement, "Indebtedness" shall mean all obligations and liabilities of
Debtor to Secured Parties under this Agreement, the Purchase Agreement and the
Notes, including any extensions, modifications or renewals thereof.  The
security interest granted hereby (the "Security Interest") shall be and remain a
                                       -----------------
first and prior security interest in all of the Collateral.

          2.  Collateral.  The Collateral that is subject to the security
              ----------
interest created hereby consists of all of the Debtor's right, title and
interest in and to: (i) all accounts receivable and (ii) all inventory, now
owned or hereafter acquired, including without limitation, all merchandise, raw
materials, parts, supplies, packing and shipping materials, work in process and
finished products including such inventory as is temporarily out of Debtor's
custody or possession or in transit and including any returns upon any accounts
or other proceeds, including insurance proceeds, resulting from the sale or
disposition of any of the foregoing and any documents of title representing any
of the above, and Debtor's books relating to any of the foregoing, and other
proceeds thereof which arise from the sale, lease, transfer or other use or
disposition of any of the foregoing or proceeds thereof and all proceeds of any
type described above acquired with cash proceeds.

          3.  Debtor's Obligation.  The Debtor shall pay to the Secured Parties
              -------------------
all amounts due and owing to the Secured Parties under the Notes both as to
principal and as to interest, in accordance with the terms of same, when and as
the same become due.

          4.  Protection of the Collateral.  Until such time as the principal
              ----------------------------
and interest of the Notes has been paid in full, the Debtor shall not create any
lien or encumbrance in the Collateral, except for any liens that are
subordinated to Secured Parties' security interest in the Collateral.

          5.  Debtor's Right to Possession.  Until default, Debtor may have
              ----------------------------
possession and beneficial use of all the Collateral and may use it in any lawful
manner consistent with this Security
<PAGE>

Agreement, provided that Debtor's right to possession and beneficial use shall
not apply to any Collateral where possession of the Collateral by the Secured
Parties is required by law to perfect the Secured Parties' security interest in
such Collateral. If the Secured Parties at any time have possession of any
Collateral, whether before or after an Event of Default, the Secured Parties
shall be deemed to have exercised reasonable care in the custody and
preservation of the Collateral if the Secured Parties take such action for that
purpose as Debtor shall request or as Secured Parties, in Secured Parties' sole
discretion, shall deem appropriate under the circumstances, but failure to honor
any request by Debtor shall not of itself be deemed to be a failure to exercise
reasonable care. The Secured Parties shall not be required to take any steps
necessary to preserve any rights in the Collateral against prior parties, nor to
protect, preserve or maintain any security interest given to secure the
Collateral.

          6.   Default.  Each of the following events shall be defined as an
               -------
"Event of Default" hereunder:

               (i)    Default in the payment of the principal or interest of the
Notes when due and Debtor fails to cure such default within five (5) days; or

               (ii)   if the outstanding amount of principal under the Notes
exceeds the sum of (1) fifty percent (50%) of the inventory as stated in the
Company's latest monthly inventory and accounts receivable report (the
"Inventory/A/R Report"), as reported to the Securied Parties, plus (ii) eighty
 --------------------
percent (80%) of the accounts receivable that are not more than 90 days past
due, net of reserves as stated in the Company's latest Inventory/A/R Report, as
reported to the Secured Parties, and the Company does not cure such default
within ten (10) days thereafter; or

               (iii)  The institution by the Debtor of proceedings to be
adjudicated a bankrupt or insolvent, or the consent by it to institution of
bankruptcy or insolvency proceedings against it or the filing by it of a
petition or answer or consent seeking reorganization or release under the
Federal Bankruptcy Act, or any other applicable federal or state law, or the
consent by it to the filing of any such petition or the appointment of a
receiver, liquidator, assignee, trustee, or other similar official, of the
Debtor, or of any substantial part of its property, or the making by it of an
assignment for the benefit of creditors generally, or the admission by it in
writing of its inability to pay its debts generally as they become due or the
taking of corporate action by the Debtor in furtherance of any such action; or

               (iv)   If, within 60 days after the commencement of an action
against the Debtor seeking any bankruptcy, insolvency, reorganization,
liquidation, dissolution or similar relief under any present or future statute,
law or regulation, such action shall not have been dismissed or all orders of
proceedings thereunder affecting the operations or the business of the Debtor
stayed, or if the stay or any such order or proceeding shall thereafter be set
aside, or if, within 60 days after the appointment without the consent or
acquiescence of the Debtor of any trustee, receiver or liquidator of the Debtor,
or of all or a substantial part of the properties of the Debtor, such
appointment shall not have been vacated.

          7.   Rights of Secured Parties.
               -------------------------
<PAGE>

          (a)  Rights as to Other Secured Parties.  Notwithstanding otherwise
               ----------------------------------
governing law with respect to the priority of security interests, all Secured
Parties shall, as between and among themselves, have the same and equal priority
in and to the Collateral, and all Secured Parties shall share the Collateral or
the proceeds thereof on a pro rata basis, based on the amount of Debtor's
outstanding Indebtedness to each Secured Party under the Notes.

          (b)  Rights as to Debtor and Collateral.
               ----------------------------------

               (i)    Upon the occurrence of any Event of Default, a majority in
interest (a "Majority in Interest") of the Secured Parties shall be entitled to
declare any Indebtedness secured hereby immediately due and payable without
notice of default, presentment or demand for payment, protest or notice of
nonpayment or dishonor, or other notices or demands of any kind or character.
In addition to the right of acceleration granted herein, and all other rights of
the Secured Parties, the Secured Parties shall be entitled to any and all
remedies available under the Uniform Commercial Code in force in the State of
California as of the date hereof, applied pro rata based on each Secured Party's
interest in the then unpaid Indebtedness.  Without limitation of any of the
foregoing, upon an Event of Default, a Majority in Interest of the Secured
Parties may, at any time and from time to time, with or without the aid or
assistance of others, enter upon the premises of Debtor and, without resistance
or interference by the Debtor, take possession of the Collateral or dispose of
any part or all of the Collateral on any such premises; or require the Debtor to
assemble and make available to the Secured Parties at the expense of the Debtor
any part or all of the Collateral at any place or time designated by the Secured
Parties which is reasonably convenient to the Debtor and the Secured Parties.
The rights of the Secured Parties hereunder shall be cumulative.  For purposes
of this Agreement, the term "Majority in Interest" means, at any time, those
Secured Parties which together hold more than 50% of the aggregate unpaid
principal and interest amount outstanding under the Notes.

               (ii)   At least a Majority in Interest of the Secured Parties
shall give Debtor notice of the time and place of any public sale of the
Collateral or of the time on or after which any private sale or other intended
disposition is to be consummated, which notice shall be mailed, by first class
mail, postage prepaid, to Debtor in the manner set forth in Section 13(b) hereof
at least ten days prior to the time of such sale or other intended disposition.

               (iii)  Each purchaser at any sale of the Collateral shall hold
the property sold absolutely free from any claim or right on the part of Debtor,
and Debtor hereby waives, to the extent permitted by law, any right which it may
have to demand a hearing or other judicial or administrative proceeding prior to
the enforcement by Secured Parties of any of their rights and remedies
hereunder. Any public or private sale of the Collateral or any part of it shall
be held at such time or times within ordinary business hours and at such place
or places as Secured Parties may fix in the notice of sale, and at any such sale
the Collateral, or the portion thereof to be sold, may be sold in one lot, as an
entirety or in separate parcels, as Secured Parties (in their sole and absolute
discretion) may determine. If permitted by law, Secured Parties may bid (which
bid may be, in whole or in part, in the form of cancellation of indebtedness)
for the purchase of the Collateral.

               (iv)   Secured Parties shall not be obligated to make any sale of
the Collateral, or any part of it, if a Majority in Interest of the Secured
Parties determine not to do so, regardless of the fact that notice of sale of
the Collateral may have been given. A Majority in
<PAGE>

Interest of the Secured Parties may, without notice or publication, adjourn a
public or private sale of the Collateral, or cause the same to be adjourned from
time to time by announcement, at the time and place fixed for sale, and such
sale may, without further notice be made at the time and place to which the same
was so adjourned.

          (c)  Inspection Rights.  Debtor shall permit the Secured Parties and
               -----------------
their designees at reasonable times and from time to time to inspect the
Collateral, wherever located.

          (d)  Fees and Costs.  Upon any Event of Default, Debtor shall pay all
               --------------
costs and expenses, including reasonable attorneys' fees, incurred by Secured
Parties in the preservation, realization, enforcement or exercise of any of the
Secured Parties' rights under this Security Agreement.

     8.   Application of the Proceeds.  All proceeds of any sale of the
          ---------------------------
Collateral by Secured Parties pursuant to Section 7 shall be applied as follows:

          First, to the payment of all fees and expenses incurred by Secured
Parties in connection with any such sale, including, but not limited to, the
expenses of taking, advertising, processing and preparing the Collateral to be
sold, all court costs and reasonable fees of counsel for Secured Parties in
connection therewith;

          Second, to the payment of accrued interest, if any, on the Notes, to
the date of receipt of such proceeds, on a pro rata basis, based on each Secured
Party's respective interest in the then unpaid Indebtedness;

          Third, to the payment of the outstanding principal balance of the
Notes, on a pro rata basis, based on each Secured Party's interest in the then
unpaid Indebtedness; and

          Fourth, to Debtor.

     9.   Further Assurances.  Upon execution of this Security Agreement, Debtor
          ------------------
and the Secured Parties shall execute, and Debtor shall promptly thereafter
file, a financing statement on Form UCC-1 with respect to the Collateral.  In
addition, at the request of the Secured Parties, the Debtor will promptly make,
execute, deliver, record, register or file all such financing statements,
continuation statements and amendments thereto, and other instruments, acts,
pledges, assignments and transfers (or cause the same to be done) and will
deliver to the Secured Parties such instruments constituting or evidencing items
of the Collateral as may be reasonably requested by the Secured Parties to
better assure them with respect to the security interests granted pursuant to
this Agreement.  The Debtor will cause all security instruments, notices and
financing statements to be duly registered, recorded and filed and to be duly
re-registered, re-recorded and refiled at the time and in the places now or
hereafter required by all applicable laws for the proper maintenance of the
validity and priority of the security interests and liens given as described
above, and will pay all fees, charges, or taxes imposed with respect to any such
registration, recording or filing.

     10.  Power of Attorney.  Each Secured Party hereby appoints irrevocably Leo
          -----------------
B. Helzel as its or his true and lawful attorney-in-fact with full power of
substitution to execute on behalf of such Secured Party a financing statement
covering the Collateral, and any and all financing statements,
<PAGE>

continuation statements and amendments thereto and all releases and termination
statements relating thereto as may be designated in writing by a Majority in
Interest of the Secured Parties.

     11.  Termination of the Security Interest.  The security interest created
          ------------------------------------
pursuant to this Agreement shall terminate upon payment, satisfaction, or
cancellation of all Indebtedness to the Secured Parties.

     12.  Miscellaneous.
          -------------

          (a)  Secured Parties may delay exercising, or omit to exercise, any
right or remedy under this Agreement without waiving that or any past, present
or future right or remedy. Neither this Agreement, nor any term hereof, may be
amended, waived, discharged or terminated except by means of an agreement in
writing signed by the Debtor and a Majority in Interest of the Secured Parties.

          (b)  All notices, requests, demands and other communications hereunder
shall be in writing and shall be deemed to have been duly given if delivered or
mailed by first-class mail, postage prepaid, to the parties at the addresses set
forth below (or such other address as shall be given in writing by either party
to the others).  Notices sent by mail shall be deemed delivered four days after
deposit with postal authorities.

          (c)  This Agreement shall bind and inure to the benefit of the
parties, their legal representatives, successors and assigns.

          (d)  This Agreement and its performance shall be governed by the laws
of the State of California, without regard to conflicts of law principles.

          (e)  This Agreement may be executed in counterparts, all of which
taken together shall constitute one and the same instrument, by signing any such
counterpart.

          (f)  This Agreement and the Purchase Agreement (and all exhibits
thereto) constitute the full and entire understanding and agreement between the
parties with regard to the subjects hereof and thereof, and no party shall be
liable or bound to any other party in any manner by any warranties,
representations, or covenants except as specifically set forth herein or
therein.
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have executed this Agreement
effective as of the date first above written.

DEBTOR:                             HEALTHCENTRAL.COM

                                    By:    /s/ C. Fred Toney
                                           ---------------------------------

                                    Title: President & CEO
                                           ---------------------------------

                                    Address:  6005 Shellmound Street
                                              ------------------------------
                                              Suite 250
                                              ------------------------------
                                              Emeryville, CA  94608
                                              ------------------------------

SECURED PARTIES                     PORRIDGE, LLC

                                    By:    /s/ Arthur J. Samberg
                                           ---------------------------------

                                    Title: Managing Partner
                                           ---------------------------------

                                    PAW OFFSHORE FUND, LTD.

                                    By:    /s/ John Ernenwein
                                           ---------------------------------

                                    Title: Chief Op. Officer of Inv. Advisor
                                           ---------------------------------

                                    PAW PARTNERS , LP.

                                    By:    /s/ John Ernenwein
                                           ---------------------------------

                                    Title: Chief Op. Officer of G.P.
                                           ---------------------------------

                                    HELZEL KIRSHMAN, L.P.

                                    By:    /s/ Lawrence B. Helzel
                                           ---------------------------------

                                    Title: General Partner
                                           ---------------------------------

<PAGE>

                                    HORNTHAL LIVING TRUST

                                    By:    /s/ James Hornthal
                                           ---------------------------------

                                    Title: Trustee
                                           ---------------------------------

                                    C. FRED TONEY

                                    By:    /s/ C. Fred Toney
                                           ---------------------------------

                                    Title:__________________________________

                                    BANK ONE TRUST COMPANY, N.A. AS TRUSTEE OF
                                    RONALD L. CHEZ IRA

                                    By:    /s/ Ruth A. Schaumberger
                                           ----------------------------------

                                    Title: Vice President
                                           ----------------------------------

                                    LEO B. AND FLORENCE HELZEL LIVING TRUST

                                    By:    /s/ Leo B. Helzel
                                           ----------------------------------

                                    Title: Trustee
                                           ----------------------------------

                                    HENRY F. BANNISTER

                                    By:    /s/ Henry F. Bannister
                                           ----------------------------------

                                    Title: __________________________________

                                   THE LEONARD AND DENA OPPENHEIM REVOCABLE
                                   TRUST DATED 1/6/00

                                    By:    /s/ Leonard A. Oppenheim
                                           ----------------------------------

                                    Title: Trustee
                                           ----------------------------------

<PAGE>

                                    LARRY ZALK

                                    By:    /s/ Larry Zalk
                                           ---------------------------------

                                    Title: _________________________________

                                    UBS PAINEWEBBER INC., NOT IN ITS CORPORATE
                                    CAPACITY BUT SOLELY AS CUSTODIAN FOR THE
                                    INDIVIDUAL RETIREMENT ACCOUNT OF CHARLES
                                    FREDERICK TONEY JR.

                                    By:    /s/ Stephen Massocca
                                           ---------------------------------

                                    Title: President
                                           ---------------------------------

                                    THE KEVIN R.E. CASTNER SEPARATE PROPERTY
                                    REVOCABLE TRUST, DATED FEBRUARY 16, 2001

                                    By:    /s/ Kevin R.E. Castner
                                           ---------------------------------

                                    Title: Trustee
                                           ---------------------------------<PAGE>

                                                                   EXHIBIT 10.48

                        AMENDMENT TO SECURITY AGREEMENT
                        -------------------------------

     This Amendment (the "Amendment") to Security Agreement is effective as of
                          ---------
May 11, 2001, by and among HealthCentral.com, a Delaware corporation (the
"Debtor"), and a Majority in Interest of the Secured Parties (as defined in the
 ------
Security Agreement), and amends that certain Security Agreement (the "Security
                                                                      --------
Agreement") dated May 10, 2001.  Capitalized terms used but not defined herein
---------
shall have the meanings given such terms in the Security Agreement.

                                   RECITALS

     A.   The Debtor and the Secured Parties are parties to the Security
Agreement, pursuant to which the Debtor granted to the Secured Parties security
interest in the Collateral (as defined in the Security Agreement).

     B.   The Debtor and a majority in interest of the Secured Parties may amend
the Security Agreement pursuant to Section 12.

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

1.   Amendment.
     ---------

     a.   Section 6(ii) of the Security Agreement is hereby amended to read in
its entirety as follows (with the amended language highlighted in bold):

                "if the outstanding amount of principal under the Notes exceeds
                 the sum of (i) fifty percent (50%) of the inventory as stated
                 in the Company's latest monthly inventory and accounts
                 receivable report, which shall reflect the Company's best
                 estimate of appropriate net adjustments and write-downs for
                 impairment (the "Inventory/A/R Report"), as reported to the
                                  --------------------
                 Secured Parties, plus (ii) eighty percent (80%) of the accounts
                 receivable that are not more than 90 days past due, net of
                 reserves as stated in the Company's latest Inventory/A/R
                 Report, as reported to the Secured Parties, and the Company
                 does not cure such default within ten (10) days thereafter; or"

     b.   Section 10 of the Security Agreement is hereby amended to read in its
entirety as follows (with the amended language highlighted in bold):

      "Power of Attorney.  A Majority in Interest of the Secured Parties may at
       -----------------
  any time in writing appoint a representative as the true and lawful attorney-
  in-fact for the Secured Parties with full power of substitution to execute on
  behalf of the Secured Parties a financing statement covering the Collateral,
  and any and all financing statements, continuation statements and amendments
  thereto and all releases and

                                      -1-
<PAGE>

          termination statements relating thereto as may be designated in
          writing by a Majority in Interest of the Secured Parties."

     2.   Counterparts.  This Amendment may be executed in two or more
          ------------
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.

                            [Signature Page Follows]

                                      -2-
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have executed this Amendment to
Security Agreement effective as of the date first above written.

DEBTOR:                             HEALTHCENTRAL.COM

                                    By:     /s/ C. Fred Toney
                                            ------------------------------------

                                    Name:   C. Fred Toney
                                            ------------------------------------

                                    Title:  Chief Executive Officer
                                            ------------------------------------

SECURED PARTIES

                                    PORRIDGE, LLC

                                    By:     /s/ Arthur J. Samberg
                                            ------------------------------------

                                    Name:   Arthur J. Samberg
                                            ------------------------------------

                                    Title:  Managing Partner
                                            ------------------------------------

                                    PAW OFFSHORE FUND, LTD.

                                    By:     /s/ John Ernenwein
                                            ------------------------------------

                                    Name:   John Ernenwein
                                            ------------------------------------

                                    Title:  Chief Op. Officer of Inv. Advisor
                                            ------------------------------------

                                    PAW PARTNERS , LP.

                                    By:     /s/ John Ernenwein
                                            ------------------------------------

                                    Name:   John Ernenwein
                                            ------------------------------------

                                    Title:  Chief Op. Officer of G.P.
                                            ------------------------------------

               SIGNATURE PAGE TO AMENDMENT TO SECURITY AGREEMENT

<PAGE>

                                    HELZEL KIRSHMAN, L.P.

                                    By:     /s/ Lawrence B. Helzel
                                            ------------------------------------

                                    Name:   Lawrence B. Helzel
                                            ------------------------------------

                                    Title:  General Partner
                                            ------------------------------------

                                    HORNTHAL LIVING TRUST

                                    By:     /s/ James Hornthal
                                            ------------------------------------

                                    Name:   James Hornthal
                                            ------------------------------------

                                    Title:  Trustee
                                            ------------------------------------

                                    C. FRED TONEY

                                    By:     /s/ C. Fred Toney
                                            ------------------------------------

                                    Name:   Ruth A. Schaumberger
                                            ------------------------------------

                                    Title:  ____________________________________

                                    BANK ONE TRUST COMPANY, N.A. AS TRUSTEE OF
                                    RONALD L. CHEZ IRA

                                    By:     /s/ Ruth A. Schaumberger
                                            ------------------------------------

                                    Name:   Ruth A. Schaumberger
                                            ------------------------------------

                                    Title:   Vice President
                                             -----------------------------------

               SIGNATURE PAGE TO AMENDMENT TO SECURITY AGREEMENT

<PAGE>

                                    LEO B. AND FLORENCE HELZEL LIVING TRUST

                                    By:     /s/ Leo B. Helzel
                                            ------------------------------------

                                    Name:   Leo B. Helzlel
                                            ------------------------------------

                                    Title:  Trustee
                                            ------------------------------------

                                    HENRY F. BANNISTER

                                    By:     /s/ Henry F. Bannister
                                            ------------------------------------

                                    Name:       Henry F. Bannister
                                            ------------------------------------

                                    Title:  ____________________________________

                                    THE LEONARD AND DENA OPPENHEIM REVOCABLE
                                    TRUST DATED 1/6/00

                                    By:     /s/ Leonard A. Oppenheim
                                            ------------------------------------

                                    Name:   Leonard A. Oppenheim
                                            ------------------------------------

                                    Title:  Trustee
                                            ------------------------------------

                                    LARRY ZALK

                                    By:     /s/ Larry Zalk
                                            ------------------------------------

                                    Name:   Larry Zalk
                                            ------------------------------------

                                    Title:  ____________________________________

               SIGNATURE PAGE TO AMENDMENT TO SECURITY AGREEMENT

<PAGE>

                                    UBS PAINEWEBBER INC., NOT IN ITS CORPORATE
                                    CAPACITY BUT SOLELY AS CUSTODIAN FOR THE
                                    INDIVIDUAL RETIREMENT ACCOUNT OF CHARLES
                                    FREDERICK TONEY JR.

                                    By:     /s/ Stephen Massocca
                                            ------------------------------------

                                    Name:   Stephen Massocca
                                            ------------------------------------

                                    Title:  President
                                            ------------------------------------

                                    THE KEVIN R.E. CASTNER SEPARATE PROPERTY
                                    REVOCABLE TRUST, DATED FEBRUARY 16, 2001

                                    By:     /s/ Kevin R.E. Castner
                                            ------------------------------------

                                    Name:   Kevin R.E. Castner
                                            ------------------------------------

                                    Title:  Trustee
                                            ------------------------------------

               SIGNATURE PAGE TO AMENDMENT TO SECURITY AGREEMENT

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