Tadalafil is an inhibitor of phosphodiesterase type 5, and is currently undergoing regulatory review in the US and in Europe. Its chemical structure is significantly different from sildenafil, and in vitro studies confirm significant potency for PDE5 inhibition, with little activity against most of the other isoforms of the enzyme including PDE6, which is the isoform of the enzyme found within the retina. The half-life of tadalafil is 17.5 hours and clinical studies suggest significant activity 24 hours post-dosing. As with sildenafil, efficacy depends upon a normal sexual stimulus, and the drug can taken be as required. Tadalafil is effective in the treatment of men with erectile dysfunction, and it appears to have a relatively mild side-effect profile, with no visual side-effects noted.