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How does caffeine work?
When we drink caffeine, the effects come on within 15 minutes. The caffeine level peaks in about an hour and for most people stays elevated for many hours. Typically, half of the caffeine is still in your body six hours later and it may take ten hours until all the caffeine is out of the bloodstream. This metabolism is variable based on different versions of the caffeine gene, CYP1A2.
Those who have variants that make them slow metabolizers will have higher caffeine levels. This means they keep caffeine around in their body for much longer than fast metabolizers who clear the stuff quickly. This explains why some people have more side effects of caffeine than others. People who are slow metabolizers have to be extra careful about caffeine consumption because they have the highest risk of cardiovascular side effects. It may even worsen their performance instead of enhance it. There are also variants of the A2a adenosine receptor gene that can make individuals more prone to anxiety with caffeine consumption than those without the variant.