Sildenafil( Viagra) is very good for patients with erectile dysfunction, however, it is contra-indicated for patients with Pulmonary veno-occlusive disease.Caution is to be excercised for patients with:1. cardiovascular disease2. uncontrolled hypertension3. if hypotensive4. if hypovolemic5. if suffering from aortic sclerosis6. if suffered from myocardial infarction or stroke in the past 6 months7. if you have bleeding disorders8. if you have impaired kidney or liver9. if suffering from retinal problems10. if you are on nitrates.Finally, it is wise to check with a doctor first before using such a medicine.Daily dose is 50 mg , taken one hour before intercourse, maximum is 100 mg per dose. and don't take more than one dose per day.