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In yesterday's discussion of headache, I deliberately left out the discussion of common pain medication for headache. If the headache is just that - a short-term disturbance of blood flow that causes an indefinite "ache" of the head, then "common" medicine is usually the best treatment.
Certainly there are rapid treatments available in the lab or hospital setting - breathing oxygen is a rapid way to cause the blood vessels in the brain to constrict, stopping the feedback loop of irritation, blood vessel dilation, inflammation and further irritation that causes the headache. For severe cases, you need to block the serotonin and histamine release, and in very severe cases, there are other neurotransmitters and chemicals that need to be blocked...
... but headache and common body aches and pains have one feature in common, and one common treatment. The common feature is inflammation, and the common treatment is aspirin or other Non-Steroidal Anti-Inflammatory Drugs (NSAIDs). Damage or stress to cells of the body - whether muscles or blood vessels - results in the release of cytokines - chemical agents that act on other cells. The common feature of cytokines is that they tend to "irritate" other cells in the body. The body reacts by causing inflammation - swelling, increased fluid in the tissue, increased blood flow, and an increase in the immune cells entering the area. One of the key inflammatory chemicals is arachidonic acid. Through the pathway at the right (figure by JFD Wolff, via Wikimedia Commons), Arachidonic acid is converted to various compounds called prostaglandins and thromboxanes. PGE2 is one of the critical agents that causes and prolongs inflammation.
The stage indicated "PGH2 synthase" is more commonly called "cylcooxygenase" or COX and there are COX-1 and COX-2 inihibitors for the two different forms of COX enzyme present. Aspirin - acetylsalicylic acid - inhibits both COX-1 and COX-2, as do most of the NSAIDs: acetaminophen/paracetamol (Tylenol), ibuprofen (Advil), naproxen (Aleve), etc. There are selective COX-2 inihibtors that appeared even more effective, since that COX-1 is less involved in inflammation, but the risk of cardiac side effects has removed them from the market.
So, the aspirin doesn't need to know where the pain is, it just needs to block this one reaction that is present wherever pain occurs. Overall, NSAIDs are the most effective relief of inflammation-related pain that we have - and it (aspirin) all started as willow tree bark.
See you next time!