[Dr.Vad]

Да, действительно длительность действия циалиса (тадалафила) оценивается в 36 ч:

Tadalafil was effective up to 36 h after dosing and was effective regardless of disease severity and causes, and in patients of all ages.

Из BJU Int. 2004 Jun;93(9):1276-81.
The efficacy and safety of tadalafil: an update.
Carson CC, Rajfer J, Eardley I, Carrier S, Denne JS, Walker DJ, Shen W, Cordell WH.

С другой стороны, действительно ли Вам нужно столь длительное действие?

другой новый препарат Левитра (варденафил) начинает действовать уже после первых 10 минут и в течение 12 ч (иногда пациенты положительно оценивают именно это качество):

1. Montorsi F, Padma-Nathan H, Buvat J, et al. Time to Onset of Action Leading To Successful Sexual Intercourse by Vardenafil (Levitra®) in Men with Erectile Dysfunction. To be presented at the 7th Congress of the European Federation of Sexology (EFS), May 12-16, 2004, Brighton, UK.

2. Stief C, Valiquette L, Montorsi F et al. Vardenafil (Levitra®) improves maintenance success rates from 15 minutes to up to 12 hours from time of dosing to start of sexual activity. Presented at the 4th World Congress on the Aging Male. 26-29 February 2004, Prague, Czech Republic.

в целом, похоже, что пациенты скорее предпочитают более новые препараты, нежели Виагру:

In terms of overall preference, most men rated vardenafil as the preferred option (47%) compared with tadalafil (19%) or sildenafil (34%) at the recommended starting dose. Vardenafil was also the preferred option at the maximum dose (43% compared to 40% taldalafil, 17% sildenafil).

"These findings highlight the benefit of having new alternatives to treat ED. This is one of the first studies to compare all three PDE5-inhibitors, and our initial results show that the majority of men who have tried all three drugs preferred the newer agents to sildenafil, and of these, more men preferred vardenafil," said Frank Sommer, MD, lead study investigator. Dr Sommer is consultant at the Department of Urology, University Medical Centre, Cologne.

Sommer F, Mathers M, Klotz T et al. Which PDE5-inhibitor do patients prefer? A comparative randomised multicenter study of sildenafil, taldalafil and vardenafil. Presented at the 6th Congress of the European Society of Sexual Medicine, Istanbul, Turkey, November 2003.