Coffee sensitivity is determined by your genetic makeup. After you drink your coffee, it only takes about 45 minutes for your body to absorb it. Caffeine is metabolised by your liver, which uses the enzyme CYP1A2 to break it down. The ability to produce this enzyme is regulated by the CYP1A2 gene. Some people produce very little of this enzyme, while others produce vast amounts. Those lacking this enzyme will be prone to having a high sensitivity to coffee.