CYP2D6: How This Liver Enzyme Affects Your Medications

When you take a pill, your body doesn’t just absorb it and call it a day. It has to break it down—mostly in the liver—and one of the main tools it uses is an enzyme called CYP2D6, a liver enzyme responsible for metabolizing about 25% of commonly prescribed drugs. Also known as cytochrome P450 2D6, it’s like a molecular scissors that cuts apart medications so your body can get rid of them safely. But here’s the catch: not everyone’s scissors work the same way. Some people have genes that make CYP2D6 super active, others have it barely working at all. That’s why two people taking the same dose of the same drug can have totally different results—one feels better, the other gets sick.

This enzyme doesn’t just handle one or two drugs. It’s involved in processing antidepressants like fluoxetine and paroxetine, pain meds like codeine and tramadol, beta-blockers like metoprolol, and even some anti-nausea drugs. If CYP2D6 is too slow, the drug builds up and you risk toxicity. If it’s too fast, the drug gets broken down before it can help. And then there are things that interfere with it—like quercetin supplements, grapefruit juice, or even some over-the-counter cold meds. These can block CYP2D6 and turn a safe dose into a dangerous one. That’s why some people end up with unexpected side effects, even when they’re following their prescription exactly.

Doctors don’t always test for CYP2D6 status, but if you’ve had bad reactions to meds before, or if something that worked for your cousin doesn’t work for you, it might be worth asking. Pharmacogenetics—the study of how genes affect drug response—is starting to show up in clinics, especially for mental health and pain management. You don’t need a fancy test to start paying attention: if you’re on multiple meds and keep running into weird side effects, CYP2D6 could be the hidden link. Below, you’ll find real-world examples of how this enzyme impacts drug safety, why some pills work for some people and not others, and what you can do to avoid dangerous interactions. This isn’t theory. It’s what’s happening in your body right now.