Phosphodiesterase type 5 inhibitors were initially approved for the treatment of erectile dysfunction in men and were later suggested for some systemic disorders, mostly due to their possible beneficial effects on endothelial functions. Paradoxically, though, phosphodiesterase type 5 inhibitors may have some life-threatening effects, for which there is weak evidence, it appears that they are associated with cardiovascular problems such as myocardial infarction and stroke. This study aimed to investigate the acute effects of tadalafil citrate on the cardiovascular system by evaluating serum oxidative status and paraoxonase-1 activity. Sera of 36 patients with erectile dysfunction were analyzed for total antioxidant status, total oxidant status and paraoxonase-1, before and after the administration of tadalafil citrate. Pre- and post-tadalafil citrate serum levels of total antioxidants, total oxidants and paraoxonase-1 were 1.1+/-0.0 and 1.6+/-0.0 micromol H(2)O(2) equiv l(-1), 10.3+/-1.1 and 6.9+/-1.2 micromol H(2)O(2) equiv l(-1), and 111.6+/-17.8 and 168.0+/-18.1 U l(-1), respectively (P<0.0001 for all results). This preliminary study confirmed that tadalafil citrate exerts a beneficial acute effect on the cardiovascular system by reducing serum levels of oxidative stress and increasing serum levels of paraoxonase-1.