Exhibit 10.5

EXECUTION COPY

AMENDMENT TO CONFIRMATION

THIS AMENDMENT (this “Amendment”) is made as of this 22nd day of March 2007,
between Credit Suisse International (“Dealer”) and ViroPharma Incorporated
(“Issuer”).

WHEREAS, Dealer and Issuer are parties to a Confirmation dated as of March 20,
2007 (the “Confirmation”) relating to Warrants on shares of common stock (par
value USD0.002 per share) of Issuer;

WHEREAS, the parties wish to amend the Confirmation on the terms and conditions
set forth in this Amendment;

NOW, THEREFORE, in consideration of their mutual covenants herein contained, the
parties hereto agree as follows:

Section 1. Terms Used but Not Defined Herein. Terms used but not defined herein
shall have the respective meanings given to them in the Confirmation.

Section 2. Amendment to the Confirmation.

 

  (a) The “Premium” under the Confirmation shall be USD51,975,000.00.

 

  (b) The “Number of Warrants” under Annex A shall be 165,624 for Components 1
through 59 and 165,647 for Component 60.

Section 3. Representations and Warranties.

Issuer represents and warrants to Dealer as follows:

 

  (a) On the date of this Amendment, (A) none of Issuer and its officers and
directors is aware of any material nonpublic information regarding Issuer or the
Shares and (B) all reports and other documents filed by Issuer with the
Securities and Exchange Commission pursuant to the Exchange Act when considered
as a whole (with the more recent such reports and documents deemed to amend
inconsistent statements contained in any earlier such reports and documents), do
not contain any untrue statement of a material fact or any omission of a
material fact required to be stated therein or necessary to make the statements
therein, in the light of the circumstances in which they were made, not
misleading.

 

  (b) Issuer is not entering into this Amendment to create actual or apparent
trading activity in the Shares (or any security convertible into or exchangeable
for Shares) or to raise or depress or otherwise manipulate the price of the
Shares (or any security convertible into or exchangeable for Shares) or
otherwise in violation of the Exchange Act.

Each of Issuer and Dealer represents and warrants to the other as follows:

 

  (c) Each of its respective representations and warranties set forth in the
Agreement and Section 7 of the Confirmation are true and correct and are hereby
deemed to be repeated as if set forth herein.

Section 4. Effectiveness. This Amendment shall become effective upon execution
by the parties hereto.

Section 5. Counterparts. This Amendment may be signed in any number of
counterparts, each of which shall be an original, with the same effect as if all
of the signatures thereto and hereto were upon the same instrument.

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Section 6. Governing Law. This Amendment shall be governed by and construed in
accordance with the laws of the State of New York.

Section 7. Effectiveness of Confirmation. Except as amended hereby, all the
terms of the Confirmation shall remain and continue in full force and effect and
are hereby confirmed in all respects.

 

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EXECUTION COPY

IN WITNESS WHEREOF, the parties have signed this Amendment as of the date and
year first above written.

 

CREDIT SUISSE INTERNATIONAL By:  

/s/ Sayedur Khan

  Name:   Sayedur Khan   Title:     By:  

/s/ Laura Muir

  Name:   Laura Muir   Title:     CREDIT SUISSE, NEW YORK BRANCH, AS AGENT FOR
CREDIT SUISSE INTERNATIONAL By:  

/s/ Yolanda Perez-Wilson

  Name:   Yolanda Perez-Wilson   Title:   Assistant Vice President   By:  

/s/ Melissa Garcia

  Name:   Melissa Garcia   Title:   Assistant Vice President  

Agreed and Accepted By:

 

VIROPHARMA INCORPORATED By:  

/s/ Michel de Rosen

  Name:   Michel de Rosen  

Title:

  President and CEO