It seems many sufferers prefer rival drugs over the Pfizer's pioneering treatment.
Cialis, a drug marketed by Lilly-ICOS, came out best in one trial.
Levitra, sold by Bayer and GlaxoSmithKline led its rivals in the other.
The studies are to be presented at the sixth congress of the European Society for Sexual Medicine in Istanbul on Monday.
Bigger and better
The results are likely to give the companies ammunition in their fierce battle for a bigger market share with Viagra, the first drug of its kind on the market that revolutionised the treatment of impotence.
Excerpts from a study of 150 patients carried out by Hamburg urologist Hartmut Porst showed that 45% of patients preferred Cialis, 30% Levitra and 13% Viagra.
The remaining 12% could not feel any measurable differences.
"The overwhelming majority of the patients prefer the two new drugs with tadalafil being ahead due to its long duration of action," the study said.
Tadalafil is the name of the active chemical in Cialis.
Another study, led by Frank Sommer at the University Medical Centre in Cologne, showed that 43% of men in a high dose, three-way preference trial preferred Levitra.
Forty percent of the patients preferred Cialis and 17% preferred Viagra.
Levitra was also found to be the preferred option at a lower, initial dose, with Viagra second and Cialis third.
Currently 86 men have completed the maximum dose trial and 47 the lower dose trial.
Viagra, Levitra and Cialis all work by blocking an enzyme called PDE-5, which affects blood flow.
Their relative merits with respect to speed of action and duration of effect are disputed, but analysts say the two new drugs have widened the market for anti-impotence treatments.