Where To buy 100mg tablets
The dose of Sildenafil citrate 100 mg is taken once per day between 30 min to 4 hours before sexual intercourse.
If the doctor prescribed you the dose of 50mg i would recommed you to split Sildenafil Citrate 100mg on two doses. Sildenafil 100mg is the cheapest option if you order the tablets online.
Last year How To Get Erections online magazine discovered the best pharmacies where one may get the ED pills. So this is the list and my comments for each:
The price for tablets 100mg is one of the best online
Price: 
83% responded it worked
Sildenafil 100mg quality:
almost 24/7
Customer support:
2 weeks to any corner of the world
Shipping:
Overnight Aid (They ship to US only via Express Mail)
Not bad
Price:
The rate of effectivenes is 78%
Sildenafil quality:
Very good support, used their toll-free several times
Customer support:
Incredibly fast, about 2-3 days when using their express service
Shipping: