These PDE5 inhibitors may cause headache, flushing, drop in blood pressure, or other adverse effects. Refills: See if you can set up automatic refills or order as needed. Ask your pharmacist for a list of the ingredients. Do not stop taking sildenafil without talking to your doctor. The main difference between them is simply the price and branding; Viagra being more expensive.Sildenafil is in a class of medications called phosphodiesterase (PDE) inhibitors. Sildenafil treats erectile dysfunction by increasing blood flow to the penis during sexual stimulation. . This medication is taken with or without food an hour before sex. It works for up to 36 hours. You can take it in a small dose daily or in a larger dose as needed. .  · Sildenafil works on the PDE5 enzyme in the lungs to relax the blood vessels. This will increase the supply of blood to the lungs and reduce the workload of the heart. .