Someone recently asked me why their prescription bottle had a little sticker on it telling them not to eat grapefruit. They had to work on me for a while but I finally let them in on a secret--pharmacists hate grapefruit farmers.
Just kidding! Grapefruit farmers are wonderful people. The actual answer is slightly less crazy (but only slightly). Grapefruit inactivates one of the types of proteins responsible for processing drugs in your gut (not your liver as I mistakenly thought when I first answered this question) and can cause a big spike in the drug concentration in your body. Unfortunately, this protein plays a big part in a lot of drugs' metabolism--about 80 drugs are affected to varying degrees.
So what does this mean for all you citrus lovers out there? If you only eat grapefruit every once in a while, check with your pharmacist. You may be able to simply monitor for increased side effects but some drugs are too sensitive to grapefruit's effects to risk even a little exposure. If you are eating a grapefruit every morning (or drinking an equivalent amount of juice) you will need to look for an alternative medication or an alternative breakfast. Have you tried muffins? They're delicious AND free of weird drug interactions!
Bonus section for the extra curious reader: there are a couple of other citrus fruits capable of causing this issue but it is extremely uncommon for anyone to eat enough of these to cause an issue.
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