Aspirin relieves headaches because of its effect on a chemical called cyclooxygenase. Cyclooxygenase is an enzyme the body uses to produce a chemical called prostaglandin. Prostaglandin is activated when the body is hurt. It tells the brain there is an injury somewhere, and the brain in turn activates the sensory response we know as pain. So, if you have an infection and your body temperature rises, causing a fever, prostaglandin tells your brain there's a problem, and your brain triggers a headache.
When you feel a headache and take an aspirin, the result is that cyclooxygenase production is inhibited, leading to a decrease in prostaglandin production and therefore a decrease in pain response.
People have used aspirin for things like headaches for more than a hundred years. In the 1980s, a group of 22,000 men, all doctors, did a little experiment. Half the subjects took an aspirin every other day, and half took a placebo. At the end of the study, twice as many members of the group taking the placebo had experienced heart attacks compared with the aspirin group [source: YHF].
People who take the recommended dose of between 80 mg and 325 mg a day have a better chance of avoiding heart attacks and, if they do have one, of surviving it [source: MedicineNet]. Why is aspirin so good for people with heart disease?
It's because it blocks cyclooxygenase and, in turn, certain prostaglandins. As it turns out, prostaglandins don't just trigger the feeling of pain. Some of them also help blood platelets clump together to form clots. Aspirin is an antiplatelet agent. It inhibits that prostaglandin that makes platelets stick together. If clots can't form in arteries, the chances of a heart attack decrease dramatically.
Much research has been done since that initial large-scale study in the '80s, and it has consistently backed up the results that brought aspirin into the heart-health arena. Studies have shown that women on a daily aspirin regimen have about 25 percent decreased risk of heart attack [source: YHF]. An American Heart Association study published in 1997 estimated that if everyone having a heart attack chewed an aspirin as soon as they started experiencing chest pain, thousands more people would survive [source: YHF]. That increased survival rate was found to be 23 percent in a study that had heart attack patients chew an aspirin within one hour of the first symptoms [source: MedicineNet]. (See Top 5 Heart Attack Symptoms That Should Have You Calling 911 to learn what to look out for.)
Aspirin isn't always as effective as the more expensive, targeted prescription drugs, but it has the advantage of being cheap, readily available and very beneficial as a preventive measure.
Still, not everyone should take daily aspirin. Some people are allergic to aspirin, and they should always avoid it. Also, since aspirin inhibits blood clotting, people with bleeding disorders and things like ulcers can't use it, because it can lead to uncontrolled internal bleeding. It can interfere with certain medications and it can cause stomachaches and other gastrointestinal problems. In high doses, it can even cause a ringing in your ears.
Because it does have these side effects, doctors usually don't recommend an aspirin regimen for healthy people who do not have disease. For people at low risk, the benefits just don't outweigh the dangers.
For more information on aspirin, heart disease and related topics, look over the links on the next page.