system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are certain fruits, such as grapefruit and cranberries, dangerous to eat whilst taking blood thinning medication like Warfarin?
gpt
Warfarin and other drugs are metabolized (broken down) into active or inactive compounds by enzymes in your liver. Grapefruit and other foods contain substances that can inhibit the activity of these enzymes, stopping the metabolism of these drugs. It can either lead to dangerously high levels of the drugs remaining in your system, or in the case of drugs that need to be broken down before they're active, dangerously low levels as the pro-drug you've taken isn't getting metabolized properly. 
Also, it can result in other enzymes acting on drugs that aren't usually active or sufficiently active, resulting in the wrong kind of metabolite forming, which can be harmful as well.