"Some people get jumpy after drinking a single cup of coffee, while others can gulp down a Venti Americano without feeling a thing. Part of that variability is due to the development of tolerance by regular coffee drinkers; but there are genetic differences in how people metabolize caffeine as well.
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https://www.23andme.com/health/Caffeine-Metabolism/
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"Coffee, CYP1A2 Genotype, and Risk of Myocardial Infarction"