Viagra works by inhibiting the PDE5 enzyme, which controls muscle contraction in the penis. 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. From Mayo Clinic to your inbox Sign up for free and stay up to date on research advancements, health tips, current health topics, and expertise on managing health. There is a myth that taking Viagra or any other ED medication will give you an instant erection, but the truth is far from it. Shipping: Discreet, fast shipping is a must. Both options require valid medical documentation, but some online services only require a health questionnaire to be filled out and reviewed for approval—meaning no face-to-face video interaction is needed. Sildenafil belongs to a group of medicines called phosphodiesterase 5 PDE5 inhibitors. · Initial dose: 5 or 20 mg orally three times a day, 4 to 6 hours apart. Maximum dose: 20 mg orally three times a day. Injection: Initial dose: or 10 mg IV bolus three times a day. Comments: No greater efficacy was achieved with oral doses higher than the maximum recommended dose. Viagra and similar forms of erectile dysfunction medication work by inhibiting PDE5 and making it easier for blood to flow to your penis and produce an erection. By inhibiting PDE5, the blood vessels that supply your penis can relax, increasing their size and allowing blood to flow more freely. When taking Viagra for erectile dysfunction, take one 50mg tablet about 1 hour before sex. The effects usually last for 4 hours, and will only start to work once you’re sexually aroused, so you should take time to relax and be intimate with your partner. If you’ve never taken erectile dysfunction (ED) before, it’s natural that you’ll.