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Lee J, Pommerville P, Brock G, Gagnon R, Mehta P, Krisdaphongs M, Chan M, Chan J, Dickson R. Physician-rated patient preference and patient- and partner-rated preference for tadalafil or sildenafil citrate: results from the Canadian 'Treatment of Erectile Dysfunction' observational study. BJU Int 2006;98:623-629.

This large study from Canada involved 2425 patients and sought to evaluate physician-rated patient preference and patient and partner-rated preference for tadalafil and sildenafil. The reason vardenafil was not included was that at the start of the 12 month study (May 2004), the drug had not been approved for licence in Canada.
Patients who planned to change treatment from tadalafil or sildenafil to the alternative drug were invited to participate in the 4-12 week study. Initially patients were assessed with background information then at the subsequent visit when the patients had been taking their new medication, they were further assessed by the physician-rated patient treatment preference questionnaire (PPPQ), and the patient and where possible the partner (295 in total) also filled out treatment preference questionnaires (TPQ).
There was excellent correlation between PPPQ and TPQ. For all categories (physicians, patients, partners) 60-70% preferred tadalafil to sildenafil whether they changed from sildenafil to tadalafil or vice-versa. Of those preferring tadalafil the reason for nearly half of them was the duration of action, and for a third of them better erections. Those who preferred sildenafil did so in two thirds of cases because of better quality erections.
This non-interventional observational study is relevant because I feel it reflects what I see in practice. Those who switch to tadalafil and stay with this drug do so because they enjoy the longer half-life and dissociating taking the tablet with the act of sexual intercourse. Those patients who prefer sildenafil having tried tadalafil do so in my experience because of a more rapid onset of action and a better quality erection. It would be interesting to see such a large study repeated incorporating vardenafil to see how it fares!