Bottomline:  Arachidonic acid , the potent omega-6 fat,  creates health-harming inflammatory chemicals in the body.  Many anti-inflammatory medications such as aspirin, Celebrex and Singulair work by blunting the effects of omega-6 fat.

Many of the health benefits derived from omega-3 fats in fish oil (EPA and DHA) are from their ability to block the harmful effects of arachidonic acid.  Arachidonic acid is the most problematic of the omega-6 fats and is found in animal products.  The plant omega-6 fat, linoleic acid, commonly found in vegetable oils (especially soybean oil, cottonseed oil, and corn oil), easily gets made into arachidonic acid by the body.

When  you eat a diet high in omega-6 fat and low in omega-3 fat, it paves the way to make more arachidonic acid.  The arachidonic acid gets holstered into heart cells, brain cells, red blood cells, lung cells and fat cells, increasing   inflammation throughout the body.  For example, these conditions are triggered by excess arachidonic acid: asthma, arthritis, headaches, cramps, pain, swelling and fever.  The medications used to treat these conditions, work by stopping this potent omega-6 fat. 

Hare are just a few example:

Aspirin—blocks arachidonic acid from making inflammatory eicosanoids, which cause pain, fever, swelling and cramping.  Also, the nonsteroidal anti-inflammatory medicines (NSAIDs), such as naproxen and ibuprofen help block the effects of arachidonic acid. (Cleland et al)

COX-2 inhibitors (such as Vioxx and Celebrex), block the COX-2 enzyme, which turn arachidonic acid into the compounds responsible for pain, inflammation and swelling. (Cleland et al)

LOX-inhibitors (such as zileuton or Zyflo), block the LOX enzyme, which turns arachidonic acid into powerful leukotriene compounds, which are responsible for asthma, allergies and possibly atherosclerosis (clogged arteries).  Leukotriene modifiers are available in the United States only by prescription for asthma and allergic rhinitis. Other leukotriene modifiers include zafirlukast (Accolate), Singulair (montelukast) and zileuton (Zyflo).  (Scow DT et al.)

Mood-Mania Stabilizers (lithium, valproate and carbamazepine), work by slowing down the effects of arachidonic acid in the brain. (Lee et al).

Statins--prevent arachidonic acid release from the LDL-cholesterol.  Arachidonic acid increases blood clots, arrhythmias and stiffening of the arteries. (Kim et al).

Comment: Many scientists believe that excess omega-6 fat in the diet is responsible for many of the chronic diseases in industrialized cultures including heart disease, cancer, mood disorder, allergies, asthma and arthritis.  They call this problem "Excessive Linoleic Acid Syndrome" or Omega-6 Fat Syndrome. (Okuyama 2007)  Notably, when arthritis sufferers lowered the omega-6 fat in their diet, in conjunction with taking fish oil--they cut their prescription medication use by 50%!  Yet, those who only took fish oil without lowerin their dietary omega-6 fat did not have these same benefits. (Cleland et al).

Links to Sources:
Cleland et al.
Fish oil: what the prescriber needs to know.Arthritis Research & Therapy 2006,(free full text)  8:202doi:10.1186/ar1876

Kim JY et al.
Lipoprotein-associated phospholipase A2 activity is associated with coronary artery disease and markers of oxidative stress: a case-control study. Am J Clin Nutr 2008 88: 630-637.

Lee et al.Antimanic therapies target brain arachidonic acid signaling: Lessons learned about the regulation of brain fatty acid metabolism.Prostaglandins, Leukotrienes and Essential Fatty Acids(77) Nov-Dec 2007:239-246. 

Okuyama, H. Omega-3 Fatty Acids Effectively Prevent Coronary Heart Disease and Other Late-Onset Diseases – The Excessive Linoleic Acid Syndrome. World Rev Nutr Diet. Basel, Karger, 2007, vol 96, pp 83-103.

Scow DT et al.
Leukotriene inhibitors in the treatment of allergy and asthma. Am Fam Physician 2007 Jan(Free full text)75(1):65-70.