Reproduced from WebMed: http://www.webmd.com/arthritis/features/pain-relief-how-nsaids-work?page=3

Pain Relief: How NSAIDs Work

NSAIDs are among the most common pain relievers in the world. And lately, they're among the most controversial. Find out what these anti-inflammatory pills actually do inside your body.

Most NSAIDs block both Cox-1 and Cox-2 enzymes. They include the over-the-counter drugs:

Other NSAIDs are available by prescription. They include:

Cox-2 inhibitors are a newer form of prescription NSAID. As you might guess, they only affect Cox-2 enzymes and not Cox-1. Two of them -- Bextra and Vioxx -- are no longer sold because of concerns about their side effects. The third, Celebrex, is still available. (see dangerous site effects)

What Are the Side Effects from Standard NSAIDs?

Most people who use NSAIDs don't have any serious problems with them. But in some -- especially those who need pain relief regularly -- there can be a downside.

When you swallow a pill, it affects your whole system, not just the part that hurts. So while an NSAID may do a great job of easing your pain, it may also be having other effects -- some of them unwanted -- in other parts of your body.

The most common risk of standard NSAIDs is that they can cause ulcers and other problems in your esophagus, stomach, or small intestine.

Why? NSAIDs prevent the creation of prostaglandins, the hormone-like chemicals that cause swelling and increase pain. But that's not all that prostaglandins do. There are actually many different types of prostaglandins in your body.

One type of prostaglandin helps protect the lining of the stomach and GI tract. And the Cox-1 enzyme helps make this prostaglandin. Since regular NSAIDs block Cox-1 enzymes, they slow down the manufacture of this prostaglandin. This is why standard NSAIDs cause high rates of gastrointestinal problems. With its defenses down, your GI tract becomes irritated and damaged by normal gastric acids.

How can NSAIDs affect your blood pressure? NSAIDs reduce the blood flow to the kidneys, which makes them work more slowly. When your kidneys are not working well, fluid builds up in your body. The more fluid in your bloodstream, the higher your blood pressure. It's that simple.

If you take NSAIDs in high doses, the reduced blood flow can permanently damage your kidneys. It can eventually lead to kidney failure and require dialysis.

NSAIDs can also cause extreme allergic reactions, especially in people with asthma. Experts aren't sure why. Many specialists recommend that people who have asthma stay away from any NSAID, especially if they have sinus problems or nasal polyps.

Most NSAIDs affect levels of both Cox-1 and Cox-2 enzymes. Cox-2 inhibitors only block the Cox-2 enzyme. So these drugs don't affect the prostaglandins that protect the lining of your GI tract. Cox-2 inhibitors offer the same pain relief as standard NSAIDs, but a much lower risk of gastrointestinal problems.

What Are the Risks of Cox-2 Inhibitors?

In a normal body, the levels of Cox-1 and Cox-2 enzymes are naturally in balance. When you block one but not the other, unexpected things can happen.

It turns out that the Cox-1 enzymes also help make a chemical that encourages blood clotting and tightens the arteries. Normally, these nasty effects are kept in check by another chemical called prostacyclin. But prostacyclin is made, in part, with the help of Cox-2 enzymes -- the enzymes that drugs like Celebrex block.

Blocking only Cox-2 upsets the balance of these enzymes. Levels of prostacyclin go down, the influence of Cox-1 goes unchecked, and your risk of heart attacks and strokes goes up.

This is why Cox-2 inhibitors have been linked to an increased risk of heart attacks and strokes. The dangers were considered so high for the drug Vioxx that it was taken off the market. Bextra, another Cox-2 inhibitor, was also removed from the market partly because of the same risk.