A Little Bit Goes a Long Way
Caffeine is known to have "vasoconstrictive" properties, which means that it narrows the blood vessels and restricts the blood flow. This is useful in relieving headache pain, since blood vessels tend to enlarge before the beginning of some headache pain, including pain associated with migraine.

Caffeine is also known as an "analgesic adjuvant," which means that it is an ingredient added to pain relievers to make them work more effectively. Quite a bit of medical research has proved that caffeine, when combined with other ingredients, can effectively treat both tension-type headache and the pain and other symptoms associated with migraine. In fact, a review of several scientific studies estimated that when caffeine is added to the common nonprescription pain relievers aspirin or acetaminophen, either alone or combined, the pain relieving effect is increased by 40%.

What does this mean for you? Some experts believe that because caffeine helps the pain relievers work better, you don't have to take as much medication to relieve your headache pain. This means you can reduce the risks of unwanted side effects, such as stomach irritation, liver disease, or even a condition known as "rebound headache."