tadalafil is taken orally, as well as sildenafil, and it is rapidly absorbed (serum levels are detectable at 20 minutes). the t (biological half-life) hovers around 17.5 hours in healthy subjects. moreover, if we consider pharmacoeconomic implications, patients that take tadalafil would pay two-thirds of the cost of sildenafil therapy. however, there are some adverse effects of this drug such as headache, diarrhea, nausea, back pain, dyspepsia, flushing and myalgia.