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From SNPedia

Caffeine is a bitter alkaloid chemical that acts as a central nervous system (CNS) stimulant. It is most commonly consumed by humans after being extracted from the cherries of the coffee plant and the leaves of the tea bush, as well as from various foods and drinks containing products derived from the kola nut. Beverages containing caffeine, such as coffee, tea, soft drinks, and energy drinks make caffeine the world's most widely consumed psychoactive substance, as well as one that is considered relatively safe and therefore legal as well as mostly unregulated.Wikipedia

Several genetic variations affect how much of a buzz caffeine will give you, as well as whether caffeine consumption may alter your risk for various diseases. In particular:

  • Caffeine metabolism: Individuals who are homozygous for the CYP1A2*1A allele are "rapid" caffeine metabolizers, whereas carriers of the variant CYP1A2*1F are "slow" caffeine metabolizers.
  • Hypertension risk:
    • Carriers of slow *1F allele are at increased risk and should thus abstain from coffee, whereas individuals with *1A/*1A genotype can safely drink coffee.[PMID 19451835]
  • Breast Cancer risk:
    • BRCA1 mutation carriers who also have at least one CYP1A2*1A (slow) allele and who drink coffee have lower cancer risk than those who don't (drink coffee).[PMID 17507615]; however, see also[PMID 18398030]
  • Breast size:
    • Among healthy premenopausal non-hormone users, 3+ cups of coffee per day was associated with lower breast volume only in CYP1A2*1F (slow) allele carriers (p=0.02), consistent with reports that coffee reduces breast cancer risk only for these allele carriers.[PMID 18813311]