You can take it in a small dose daily or in a larger dose as needed. You’ll soon start receiving the latest Mayo Clinic health information you requested in your inbox. As with sildenafil, a high-fat meal or alcohol keep the body from absorbing it as quickly. The most common side effects of VIAGRA: headache; flushing; upset stomach; abnormal vision, such as changes in color vision such as having a blue color tinge and blurred vision; stuffy or runny nose; back pain; muscle pain; nausea; dizziness; rash. Erectile dysfunction is a condition where the penis does not harden and expand when a man is sexually excited, or when he cannot keep an erection. This medication is taken with or without food an hour before sex. Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. · Sildenafil is used to treat men who have erectile dysfunction (also called sexual impotence). Sildenafil belongs to a group of medicines called phosphodiesterase 5 (PDE5) inhibitors. These medicines prevent an enzyme called phosphodiesterase type-5 from working too quickly. In combination with sexual stimulation, sildenafil works by increasing blood flow to the penis to help a man get and keep an drug does not protect against sexually transmitted.  · Sildenafil under the name Viagra is used to treat erectile dysfunction (impotence) in men. Another brand of sildenafil is Revatio, which is used to treat pulmonary arterial hypertension and improve exercise capacity in men and women.