Viagra (Sildanafil) is not addictive. If you have Erectile Dysfunction (ED) then you take one Viagra blue pill about an hour before you are expecting 'action'! They generally last about 12 hours and must be taken once you have been cleared by a doctor as they can alter blood pressure.
Ciallis (Taladafil) is similar in operation but can last 36 hours (the effect, not the erection!!)
Both these medicines can be taken as and when needed.
This information is not from a medical source and as such should not be relied upon.