Sildenafil and similar drugs have been used for several years to treat erectile dysfunction. It has been noticed that in some people, sildenafil causes a subtle increase in the length of time that visual images that we see are retained by the retina. It is thought that this might be due to an effect of sildenafil on inhibiting an enzyme called phosphodiesterase type 6 (PDE6) which is present in the retina. By giving single oral doses of sildenafil and a similar drug called tadalafil which has less effect on PDE6, we hypothesise that this is the mechanism of the change in vision caused by sildenafil. By performing computerised visual test, we plan to compare the effects of sildenafil, tadalafil and placebo tablets on vision in healthy volunteers.

Inclusion Criteria:

• Healthy
• Male
• 18-55 years

Exclusion Criteria:

• Significant medical or psychiatric illness
• Cardiac disease
• Hyper or hypotension
• Renal disease
• Liver disease
• Stroke
• Sickle cell anaemia
• Multiple myeloma
• Leukaemia
• Bleeding disorders
• Peyronie's disease
• Priapism
• Subjects receiving prescribed medications
• Subjects with known visual abnormalities other than refractive errors, including retinitis pigmentosa, optic neuropathy