Common ones include: Flushing of the skin. It works for up to 36 hours. Be sure the one you choose works in your state. Now, the Drug Enforcement Agency DEA has proposed new rules that will require an in-person visit for anything more than a day supply of controlled substances to be filled by an online pharmacy. People taking nitrates like nitroglycerin Nitrostat or isosorbide mononitrate Imdur should not take PDE5 inhibitors, as doing so could result in a dangerous drop in blood pressure. This increase in blood flow results in a firmer, longer-lasting erection.Sildenafil (Viagra) is a phosphodiesterase-5 (PDE-5) inhibitor. PDE-5 is a protein in your body that breaks down the molecules that normally cause an erection. When it blocks . Sildenafil generic Revatio and Viagra Used for Pulmonary Hypertension and Erectile Dysfunction MORE expand_more Check your prescription Prescription 20mg sildenafil . Best for generic Viagra and other health and wellness medications: Lemonaid Sildenafil. Best.