Frequency of Viagra Use

  Pfizer's labelling for Viagra specifies no more than once daily usage, with a maximum daily dose of 100mg.  If a patient is using the 25 or 50mg tablets, one would have thought that if one tablet does not elicit the desired response after an hour or so, than another one may be taken, as long as the total dose does not exceed 100mg.
  Is this correct?  Or is there a specific reason for Pfizer to specify "once daily", other than their clinical studies were limited to this type of dosing?
  Thanks for the assistance.
  Cheers,
  Pete, Pharm.D.
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Dear Pete,
From my discussions with Pfizer, the doses and frequency of administration recommended were based on the studied doses. That one can dose additionally during the same sexual episode is unknown to the company.
Incidentally the lead article in  the latest

[JohnConnor12]

A related discussion, Functioning of Viagra was started.