Sildenafil and Viagra are the same drug. Viagra is the original branded version first launched in 1998 and Sildenafil it's cheaper generic version available in The UK since 2013.
Both drugs are taken around one hour before sexual activity and the treatment remains in the blood stream for 4 hours. The onset of action can be delayed if consumed with a meal.
The side effects of Sildenafil and Viagra are usually mild with the treatment being well tolerated and effective. They both come in 25mg, 50mg and 100mg strengths.
The starting dose is 50mg to be taken one hour before sexual activity
Most men will find that the 50mg starting dose has the desired effect, but if it does not work on four separate occasions, the dose should be increased to 100mg (provided there are no side effects). If side effects are felt on 50mg, then a 25mg dose should be used the next time.
Sildenafil or Viagra should be used on multiple and separate attempts before considering it is not effective for you.