How does Sildenafil work?
Sildenafil's active ingredient (sildenafil citrate) increases blood flow to the penis. It enables a firmer, longer-lasting erection if you are sexually aroused.
The treatment will not resolve the underlying physical cause of your ED or increase your sexual desire.
How effective is Sildenafil?
Sildenafil treats ED in around two thirds of cases. If you have eaten a large meal or drunk more than 1-2 units of alcohol, you may find Sildenafil is less effective.
We advise trying an ED treatment 8 times before giving up, considering a higher dose or seeking an alternative.
How to take Sildenafil
Take one Sildenafil tablet an hour before sex. Sildenafil works best on an empty stomach. Sildenafil can be taken once a day. You should never take more than one Sildenafil tablet in 24 hours. Do not combine Sildenafil with other ED medicines.
Does Sildenafil work the first time?
It’s thought that Sildenafil works for as many as two thirds of men. However, there’s a chance it might not work if it’s your first time taking Sildenafil. We recommend you try an ED medication up to 8 times on separate occasions before deciding if it works for you or not. 
 
Sildenafil (generic Viagra)
Sildenafil tablets contain the same active ingredient as Viagra (sildenafil citrate). Sildenafil is a generic, common ED treatment and is as effective as branded Viagra.  
Effective for 62-82% of men
Works in 1hr and lasts 4-5hrs
Works in the same way as Viagra