Zenegra (sildenafil citrate) is a drug whose history goes back to early 90s, with the safety profile being proven by dozens of patient-years.
Phosphodiesterase inhibitors, to which sildenafil belongs, are drugs with the mechanism promoting the vasodilation of the corpus cavernosum and therefore the erection. Therefore, they are vascular drugs. And as such, they must be well managed, with patients being careful about the doses and the general use.
It is true that these drugs, being vasodilators (and in fact among the side effects, in healthy subjects, it will be possible to observe facial flushing and headaches both from vasodilation) may lower the pressure a little. This helps to understand why one must have a certain caution in particular if taking nitrates, drugs which will strengthen the already robust hypotensive effects making their simultaneous assumption incompatible.