Brand names Why is this medication prescribed? Also tell your doctor if you or any of your family members have or have ever had an eye disease such as retinitis pigmentosa an inherited eye condition that causes loss of vision or if you have ever had sudden severe vision loss, especially if you were told that the vision loss was caused by a blockage of blood flow to the nerves that help you see. Take sildenafil exactly as directed. Your doctor may need to change the doses of your medications or monitor you carefully for side effects. If you have erection problems sildenafil works by temporarily increasing blood flow to your penis when you get sexually excited.The recommended dosage of Viagra for erectile dysfunction (ED) is 50 mg, taken as needed about 1 hour before sexual activity. You can take it 30 minutes to 4 hours before sexual activity. And you. Cialis lasts up to 36 hours in the body, while Viagra lasts 4–6 hours in the body. Both are taken 30 minutes before sex. Cialis can cause limb pain, while Viagra can cause vision change, rash. Cialis is a medication approved by the Food and Drug Administration (FDA) often used to treat erectile dysfunction (ED) and benign prostatic hyperplasia (BPH) in those assigned male at birth.