You’ll soon start receiving the latest Mayo Clinic health information you requested in your inbox. Some medications should not be taken with sildenafil. This medication is taken with or without food an hour before sex. Sildenafil may cause side effects. Other uses for this medicine This medication is sometimes prescribed for other uses; ask your doctor or pharmacist for more information. Its effects might last longer if you have mild to moderate ED.Common Viagra side effects may include: flushing (warmth, redness, or tingly feeling); headache, dizziness; abnormal vision (blurred vision, changes in color vision) runny or . For treatment of erectile dysfunction: For oral dosage form (tablets): Adults up to 65 years of age—50 milligrams (mg) as a single dose no more than once a day, 1 hour before sexual . Oral medicines for ED usually work well and cause few side effects. The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil .