Whenever I take viagra (sildenafil) I start to get dizzy and black out. Is this a normal side effect of this medication?

Stop taking med! You mean this has happened more than once? And you haven't talked to your prescribing doc about this? Or sought medical attention? Sildenafil (Viagra) was originally studied as a potential treatment for high blood pressure. However, it wasn't very effective in that role but it can still cause low blood pressure & even passing out especially when taken w/nitrates. Best to stop & discuss w/your doc.