Aortic stenosis, hypertrophic obstructive cardiomyopathy, pre-existing CV risk factors, multiple system atrophy manifesting as severely impaired autonomic control of BP. Angulation, cavernosal fibrosis or Peyronie's disease, sickle cell anaemia, multiple myeloma or leukaemia. Bleeding disorders or active peptic ulceration. Concomitant w/ other drugs for erectile dysfunction, ritonavir, α-blockers, Na nitroprusside. Galactose intolerance, Lapp lactase deficiency, glucose-galactose malabsorption. May affect ability to drive or operate machinery. Renal & hepatic impairment. Childn <18 yr.