Stop taking VIAGRA and call your healthcare provider right away if you have any sudden vision loss sudden hearing decrease or hearing loss. Some medications should not be taken with sildenafil. During 3 to 6 months of double-blind treatment or longer-term 1 year , open-label studies, few patients withdrew from active treatment for any reason, including lack of effectiveness. The information provided is for educational purposes only and is not intended to replace discussions with a healthcare provider. If you are taking sildenafil for erectile dysfunction, your doctor will probably start you on an average dose of sildenafil and increase or decrease your dose depending on your response to the medication. Cialis reaches its maximum effectiveness two hours after you take it.Key facts. Sildenafil (Viagra) is a medicine used to treat erectile dysfunction (ED, also called impotence). Sildenafil works by helping to relax the blood vessels in the penis, allowing more blood to flow in and cause an erection. You should take sildenafil about an hour before planned sexual activity, so it has time to work. The Answers You Need. Expert-reviewed resources to help you navigate your health journey, from A to Z and everything in between. Compare prescription drug prices and . VIAGRA is available by prescription only in 25 mg, 50 mg, and mg tablets. Patients should always ask their doctors for medical advice about adverse events. You are .