You use Viagra 100 mg for ED when lower doses (usually 50 mg) haven’t worked well enough and your doctor says a higher dose is appropriate. It helps whether your ED is vascular, diabetic, post‑prostatectomy, psychogenic, or mixed. In clinical studies, sildenafil improved erectile function in most men (roughly 60–80% responded vs. 20–30% on placebo), allowing you to get and keep an erection long enough for intercourse when sexually stimulated.