"According to the results of a case-control study reported in 2006, coffee is the most widely consumed stimulant in the world. The caffeine in coffee is primarily metabolized by the cytochrome P450 1A2 in the liver, accounting for 95% of metabolism. Those with the *1F gene variant are slow caffeine metabolizers, and individuals with two copies of the *1A/*1A genotype are rapid caffeine metabolizers.

The results column gives the three possible alleles from the test"

AA Fast caffeine metabolism
CC Slow caffeine metabolism
AC Slow caffeine metabolism

Link: http://www.ncbi.nlm.nih.gov/pubmed/16522833

Intake of coffee was associated with an increased risk of nonfatal MI only among individuals with slow caffeine metabolism, suggesting that caffeine plays a role in this association."

MI = Myocardial Infarction

I suppose I should stop drinking coffee!