When is the drug hydralazine used?

Few reasons. It's a vasodilator (dilates blood vessels) and has a couple of common uses 1: it can be used for high blood pressure, although usually not a first choice these days since its dosed frequently when given in pill form. 2: it's used in heart failure. It's called an after-load reducer. Basically makes it easier for a weakened heart (heart failure) to push blood out to the body.