Recent heart or blood vessel problems. You can buy it from a store-based pharmacy or reputable online pharmacy with a prescription. Based upon measurements of sildenafil in semen of healthy volunteers 90 minutes after dosing, less than 0. It can be dangerous to take sildenafil if you also take medicines called nitrates often given for chest pain. By controlling phosphodiesterase, sildenafil helps to maintain an erection that is produced when the penis is physically stimulated. They’re often the first line of treatment for trouble getting or keeping an erection, called erectile dysfunction ED.Cialis lasts up to 36 hours in the body, while Viagra lasts 4–6 hours in the body. Both are taken 30 minutes before sex. Cialis can cause limb pain, while Viagra can cause vision change, rash. Sildenafil (Viagra) is a medicine used to treat erectile dysfunction (ED, also called impotence). Sildenafil works by helping to relax the blood vessels in the penis, allowing more blood to flow in and cause an erection. You should take sildenafil about an hour before planned sexual activity, so it has time to work. Viagra costs around $60–$70 per tablet. Generic sildenafil can range from $2–$18 per pill, depending on the manufacturer and dose. Efficacy and safety of sildenafil by age in men with.