Re: What is in aspirin that makes it effective?

The active ingredient in aspirin is acetylsalicyacetic acid (ASA).  It 
works by chemically modifying an enzyme which is responsible, in part, for 
causing fever and inflammation.  The enzyme is known by several names in 
the scientific literature, including:  cyclooxygenase (COX), prostaglandin 
synthase (PGS).  It turns out that there are two closely related forms of 
this enzyme, so they are now numbered:  COX-1 and COX-2 or PGS-1 and PGS-2.  
The acetyl group in ASA reacts with a permanently binds to the active site 
of these enzymes, such that they no longer work.  However, over a period of 
several hours, most cells make new enzyme which does work.  That is why you 
have to keep taking aspirin for the effect to last.  

So I have not really explained how these two related enzymes cause fever 
and inflammation.  Well it turns out that the second form of the enzyme 
(i.e., COX-2 or PGS-2), is turned on during an infection or injury.  This 
enzyme is responsible for the generation of compounds known as 
prostaglandins (thus the name prostaglandin synthase).  It is the 
production of prostaglandins (PG) in the brain that induce fever.  PG made 
elsewhere in the body, let's say during an infection, promote local 
inflammation and swelling.  This is meant to promote the activation of 
immune cells which are needed to fight the infection.  However, too much 
inflammation can be bad (i.e., it cause pain!). 

Finally, I thought you might like to know why too much aspirin is bad for 
you, at least bad for your stomach.  As the aspirin dissolves in your 
stomach it blocks the COX-1 enzyme in the cells lining the stomach.  This 
COX-1 is needed to promote the secretion of mucous that is responsible for 
protecting your stomach lining from the acid in your stomach.  Too much 
aspirin or frequent use can cause irritation of the stomach lining and 
eventually may lead to a bleeding ulcer.  Ouch!  

The drug companies have been developing new drugs that work on COX-2, but 
leaves COX-1 alone.  In theory these drugs should help reduce pain and 
inflammation without the bad side effects of aspirin. A few of these new 
anti-inflammatory drugs are out in the marketplace now, but they start out 
as prescription drugs initially, so you may not have read much about them.