Rarely reported side effects include: an erection that will not go away priapism. The mean steady state volume of distribution Vss for sildenafil is L, indicating distribution into the tissues. As in the other titration studies, patients were started on 50 mg and allowed to adjust the dose up to mg or down to 25 mg of VIAGRA; all patients, however, were receiving 50 mg or mg at the end of the study. A starting oral dose of 25 mg should be considered in those patients [see Dosage and Administration 2. Side effects Side effects of oral ED medicines are often mild. In these studies, involving about patients, analyses of patient diaries showed no effect of VIAGRA on rates of attempted intercourse about 2 per week , but there was clear treatment-related improvement in sexual function: per patient weekly success rates averaged 1.Q&A. Drug Images. What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a prescription drug that’s used in adult males* . 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. How can I buy sildenafil? You need a prescription from a doctor to obtain sildenafil. You can buy it from a store-based pharmacy or reputable online pharmacy with a prescription. Some people buy sildenafil online without a prescription. This isn’t a good idea, as you can’t be sure whether these pills contain the medicine or are counterfeit.