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Tolerance to the Therapeutic Effect of Tadalafil Does Not Occur During 6 Months of Treatment: A Randomized, Double-Blind, Placebo-Controlled Study in Men with Erectile Dysfunction.
McMahon, Chris G. MB, BS, FAChSHM *; Carson, Culley C. MD, FACS +; Fischer, Christopher J. BS ++; Wang, Wei Christine MS [S]; Florio, Vincent A. PhD ++; Bradley, John D. MD [S]
[Article] Journal of Sexual Medicine. 3(3):504-511, May 2006.

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Introduction: Tolerance can cause a decrease in drug efficacy during chronic therapy, possibly leading to treatment failures.

Aim: The aim of this article is to determine whether tolerance developed to the effects of tadalafil on erectile function (EF) over a 6-month treatment period.

Methods and Main Outcome Measures: Post hoc analysis of data from a multicenter, double-blind, randomized, placebo-controlled, parallel group study was performed. Men (>=18 years of age) with erectile dysfunction (ED) were randomized to treatment with placebo (N = 47) or 20-mg tadalafil (N = 93) taken as needed for 6 months. This report focuses on efficacy assessed with the Sexual Encounter Profile (SEP) diary (diaries were collected after a 4-week treatment-free run-in period [baseline], and monthly for 6 months), and with the International Index of Erectile Function (IIEF) (administered at baseline, and at 3 and 6 months).

Results: The mean per-patient percentage "yes" response on SEP question 3 (SEP3, successful intercourse) was 33 /- 4% at baseline, 74 /- 4% after 1 month, and 78 /- 4% after 6 months of tadalafil treatment. The IIEF EF domain score was 16.2 /- 0.7 at baseline, 24.3 /- 0.8 after 3 months, and 24.3 /- 0.9 after 6 months of tadalafil treatment. In a subgroup of patients who took tadalafil >=3 times per week (N = 24), the SEP3 score was 87 /- 4% after 1 month and 93 /- 3% after 6 months of treatment, and the IIEF EF domain score was 27.3 /- 0.9 after 3 months and 28.5 /- 0.4 after 6 months. Of 16 tadalafil-treated patients who discontinued, three cited a lack of efficacy.

Conclusions: Tadalafil treatment significantly improved SEP3 and IIEF EF domain scores. The efficacy of tadalafil, taken as needed, was maintained over a 6-month treatment period in men with ED.