Kidney disease that needs to be treated with dialysis. Sildenafil and Viagra both work by inhibiting the PDE5 enzyme, which controls muscle contraction in the penis. These include a heart attack, stroke or a serious heart rhythm disorder in the past six months. The main difference between them is simply the price and branding; Viagra being more expensive. You can take it in a small dose daily or in a larger dose as needed.Generic: Sildenafil. Form: oral tablet; Strengths: 25 mg, 50 mg, mg; Brand: Viagra. Form: oral tablet; Strengths: 25 mg, 50 mg, mg; Adult dosage (ages . Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. They enhance the effects of a chemical the body makes that relaxes .  · a light-headed feeling, like you might pass out. Common sildenafil side effects may include: flushing (warmth, redness, or tingly feeling); headache, dizziness; .