Sildenafil tablets for erectile dysfunction come in different strengths ranging from 25mg to 100mg. The usual dose is 50mg when you need it. Do not take it more than once a day. The dose can be increased to 100mg or decreased to 25mg, depending on its effect. Take sildenafil up to 4 hours before you want to have sex. For sildenafil to work properly, you'll need to be sexually excited.