I have blogged about aspirin many times dating back to September 19, 2009. Yes I have been doing this for a long time. What is old is new again in an article published in Circulation online last week. Entitled “Drug Resistance and Pseudoresistance: An Unintended Consequence of Enteric Coating Aspirin” (DOI: 10.1161/CIRCULATIONAHA.112.117283), this article helps to dispel yet another Urban Legend, that of the concept that certain people do not respond to aspirin appropriately.
This is actually a big deal if it was true. Taking aspirin after a myocardial event decreases your risk of death by 20%. Like the discussion of “more taste, less filling” or the tooth fairy, you either believe in aspirin resistance or you don’t. A great deal of money is spent on testing for something that most people feel doesn’t exist. This article is just one in a long line of articles. This article also helps disspell another myth, that of enteric coating.
Just because it should work doesn’t mean it does work. Enteric coating was promulgated to reduce the risk of GI bleeding. This “proof” was, however, incorrectly done. What enteric coating does is reduce the risk of visible stomach lining lesions. The proper study, never done, would have been to assess for all bleeding not only for those visibly seen. This is because the bleeding is based on the prostaglandin function of aspirin and not the irritative function of aspirin. You can get GI bleeding from aspirin by giving it rectally. Similarly, Plavix causes GI bleeding and it is not based on an irritative basis.
What is the science? We believe that approximately 50 mg of aspirin is needed to inhibit the platelet receptor in question. This, however, requires the aspirin to be perfectly absorbed from the stomach. This definitely does not happen all the time. Therein lies the problem with the enteric coating. When the 400 healthy subjects in this study were given enteric coated aspirin, none were found to be aspirin resistant by common tests. 49% did not have proper activation at 4 hours, and this dropped to 17% at 8 hours. Of the 17%, after one week of aspirin therapy, none were found to be resistant.
What have we learned? Enteric coating does little to protect you from GI bleeding in aspirin use. It does however interfere with the acute use of aspirin such that in emergency situations, regular aspirin should be used and preferably chewed. It does not seem to help if you chew enteric coated aspirin. Good luck with finding regular aspirin in a hospital, however, since most – if not all – of the aspirin has been replaced with the enteric coated aspirin. Long term administration of any form of aspirin eradicates the apparent resistance.
The naysayers have already appeared. This study utilized healthy people, and their conjecture is that “sick” patients are different. This may or may not be true. It has yet to be determined but probably never will be because it takes time and money to do this work, and it is unlikely to get funding.
I can guarantee we have not seen the last on this topic. Over 150 years and we still do not understand the “simple drugs.”