It works by dilating the arteries in your body. This has the effect of lowering your blood pressure. If you are taking anything else that has the same effect, there is an additive effect and your blood pressure will drop more suddenly. So, for example, any drug that contains a nitrate or has been prescribed to treat high blood pressure can be dangerous. Do not forget this includes viagra. Each dose is intended to be taken once-daily. If you take two or more closely together, your blood pressure will drop. Alcohol also has a hypotensive effect (it lowers your blood pressure). In a young, otherwise healthy man, a little alcohol is not a danger. But excessive drinking can become as dangerous as alpha-blockers, nitrates, etc. At the very least, these combinations will make you dizzy. At worst, they produce unconsciousness.