Sildenafil inhibits phosphodiesterase-5, the enzyme that breaks down cGMP in penile tissue. The result: enhanced vasodilation in response to arousal. Onset 30-60 min, duration ~4 hours.
Dose: 25-100mg as needed, with arousal. Take on empty stomach for fastest onset.
Caveats: Nitrate drug interaction is dangerous (severe hypotension). Headache, flushing, nasal congestion are common. Doesn't cause arousal. Needs sexual stimulation to work.