Before starting, stopping, or changing any medications while taking sildenafil, please get the advice of your doctor or pharmacist. Tell your doctor if sildenafil is not working well or if you are experiencing side effects. What should I do if I forget a dose? The best time to take sildenafil is about 1 hour before sexual activity, but you can take the medication any time from 4 hours to 30 minutes before sexual activity. If you prefer, you may contact the U.Is generic Viagra (sildenafil) available? How long does it last? It really depends based on the person. Most of the time, Viagra starts to work after 30 minutes. Some people note that Viagra. Side Effects. Dizziness, headache, flushing, or stomach upset may occur. Vision changes such as increased sensitivity to light, blurred vision, or trouble telling blue and green colors apart may. Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in adults (Liqrev, Revatio) and children 1 year of age and older (Revatio) with pulmonary arterial hypertension (PAH; high blood pressure in the vessels carrying blood to the .