Hydralazine has a direct vasodilatory effect on vascular smooth muscle; the predominant effect is on arterioles with little effect on veins. Arteriolar vasodilation results in reflex sympathetic stimulation.

Adverse reflex effects limit the effectiveness of hydralazine when used alone. When used in combination with a diuretic and a beta blocker, these effects are minimised. Hydralazine has been associated with the development of a lupus-like syndrome. Prolonged treatment for longer than six months, and daily doses of greater than 100 mg, are associated with an increased risk of developing the syndrome. It is also more likely in women and in slow acetylators. If treatment is prolonged and daily doses are in excess of 100 mg, the acetylator phenotype should be determined and antinuclear factor checked.