Online pharmacies don’t always hold up to scrutiny, either. Next review due on August 01, In psychogenic erectile dysfunction, psychological issues prevent maintaining or getting an erection — nothing physical. For example, cocaine constricts blood vessels, and sildenafil citrate does the opposite. If you don’t have a prescription, the drug itself — not just how you use it — may be a problem. The danger comes from taking it with other drugs or getting it from someone without a medical license. · Before taking this medicine. You should not use sildenafil if you are allergic to it, or: if you take other medicines to treat pulmonary arterial hypertension, such as . 1 day ago · Have an accurate diagnosis and prescription: Remember that Viagra and generic sildenafil are not available over the counter — you will need a prescription. . Generic name: sildenafil (oral) [ sil-DEN-a-fil ] Brand names: Revatio, Viagra Drug class: Impotence agents Medically reviewed by Sophia Entringer, PharmD. .