Sildenafil is prescribed for use as needed. It generally takes about an hour before becoming effective (in the usually prescribed pill form), so a little extra patience (and planning) must be taken into consideration. Some patients report chewables starting to work faster in their system.  Sildenafil wears off after four to six hours.

For men that want to be able to have sex at any time, Tadalafil might be the better choice.  It is advised to be taken daily.  Since it stays in your system for 24-36 hours, it gives you more opportunities for spontaneous intercourse whenever you and your partner are in the mood.

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