Sildenafil is part of a group of drugs known as PDE5 inhibitors. Shake the liquid well for 10 seconds before each use to mix the medication evenly. However, all medications come with risks. Importantly for you, this means the way both drugs work, how long they take to work, how well they work and how long they work for, are the same. Your doctor may need to change the doses of your medications or monitor you carefully for side effects. · 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. . 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. .