The medicine will come with a plastic syringe or spoon to help you take the right amount. For erectile dysfunction, it’s best to take sildenafil on an empty stomach. In vivo studies: No significant interactions were shown with tolbutamide mg or warfarin 40 mg , both of which are metabolized by CYP2C9. Proper Use Use sildenafil exactly as directed by your doctor. Take sildenafil as needed before sexual activity.Viagra is available in three strengths: 25 milligrams (mg), 50 mg, and mg. What are the typical dosages of Viagra? Typically, your doctor will start you on the recommended . Sildenafil tablets for erectile dysfunction come in different strengths ranging from 25mg to mg. The usual dose is 50mg, when you need it. Do not take it more than once a day. The dose can be increased to mg or decreased to 25mg depending on its effect. Take sildenafil up to 4 hours before you want to have sex. Maximum dose: 20 mg orally three times a day. Injection: Initial dose: or 10 mg IV bolus three times a day. Comments: No greater efficacy was achieved with oral doses higher .