Strokes come in two varieties, hemorrhagic (bleeding) and occlusive (blocking).
The bleeding variety occurs when a blood vessel in the brain ruptures. They can be prompted by high blood pressure, aneurysms or by substances that interfere with clotting like aspiring or large doses of alcohol. The blocking variety is analogous to a heart attack in the brain where artery blockage occurs.
Studies show that moderate alcohol consumption results in a very small (some studies say no) effect on the hemorrhagic variety and may possibly decrease the occlusive variety which occurs about 10 times more frequently.
A recent study, conducted by a team led by Dr. Arthur L. Klatsky, M.D., at the Kaiser Permanente Medical Center in Oakland, Calif. (Stroke, 1989) concluded that, "Daily consumption of three or more drinks, but not lighter drinking was related to higher hospitalization rates for hemorrhagic cerebrovascular disease."
Their data indicated that people who consumed one to two drinks per day had a 0.75 relative risk of being hospitalized for hemorrhagic stroke compared to abstainers (RR = 1.00). Those drinking three or more drinks per day had a relative risk of 1.38.
The Klatsky study also found an indication that moderate alcohol consumption offered some protection against occlusive strokes. Their data indicated that people consuming one to two drinks per day had a relative risk of occlusive stroke of 0.37 compared with abstainers (RR = 1.00). People consuming three or more drinks per day had a relative risk of 0.50.
The Klatsky paper cautions that, "A single study cannot establish an inverse relation between alcohol use and cerebrovascular disease." Since then, several other studies have shown a similar relationship.
The similarity between ischemic heart disease and occlusive strokes makes it plausible that this inverse relationship will continue to be confirmed by future studies.