Your doctor may need to change the doses of your medications or monitor you carefully for side effects. This finding is consistent with the inhibition of PDE6, which is involved in phototransduction in the retina. 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. Sildenafil does not prevent pregnancy or the spread of sexually transmitted diseases such as human immunodeficiency virus HIV. Figure 6 shows that regardless of the baseline levels of function, subsequent function in patients treated with VIAGRA was better than that seen in patients treated with placebo. If you ever need emergency medical treatment for a heart problem, the healthcare providers who treat you will need to know when you last took sildenafil. You need to be sexually excited for it to work.Oral medications, like Viagra and Cialis, which require a prescription and are commonly used to treat ED, can take 30 minutes to take effect, she adds. Injections take less time . Shop for Viagra in Sexual Wellness Top Brands. Buy products such as Promensil Natural Relief For Menopausal Symptoms Tablets – 30 Ea, 6 Pack at Walmart and save. This medicine is available on prescription and you can also buy it from most pharmacies. It comes as standard tablets, chewable tablets, and as a liquid that you swallow. Key facts. It usually takes 30 to 60 minutes for sildenafil to work for erectile dysfunction. You can take it up to 4 hours before you want to have sex.