If you’re looking to extend the caffeine buzz from your  daily coffee , a taste of grapefruit might do the trick. Grapefruit contains a chemical that interacts with other substances, which is why doctors caution against  pairing it with dozens of medications . The chemical works by blocking an enzyme in the digestive system that helps break down certain substances, allowing more of the drug to enter the bloodstream. Drug concentrations in the blood can rise too fast and to dangerously high