Metabolized: What Happens to a Drug After You Swallow It

Ever wonder why one pill works for your friend but not for you? A lot of that comes down to how the body metabolizes drugs. Metabolism is the chemical process—mostly in the liver—that changes a medicine so it can do its job, then leave your body. Some drugs get broken down fast, others slowly, and some aren’t changed at all. That affects how strong they are, how long they last, and what side effects you might get.

Key things to know about drug metabolism

Most drug metabolism involves enzymes called CYP450 (think CYP3A4, CYP2D6, CYP2C9). These enzymes act like gatekeepers. For example, simvastatin is mainly handled by CYP3A4, so grapefruit juice — which blocks CYP3A4 — can raise simvastatin levels and increase side effects. Codeine needs CYP2D6 to turn into morphine; people who make lots of that enzyme (ultra-rapid metabolizers) can get too much effect and risky breathing problems. On the flip side, metformin isn’t metabolized much — it’s mostly excreted by the kidneys — so kidney problems matter more than liver ones for that drug.

First-pass metabolism is another big idea. When you swallow a pill, the gut and liver can break a lot of it down before it reaches the bloodstream. That’s why some medicines are given under the tongue or by injection — to avoid the first pass. Nitroglycerin is a classic example: taken sublingually so it works fast.

Practical tips you can use

1) Tell every doctor and pharmacist what you take. Even herbal stuff like St. John’s wort speeds up some enzymes and can reduce birth control or antidepressant levels. 2) Ask about food interactions. Grapefruit is a common culprit; dairy or high-fat meals can also change absorption for certain drugs. 3) Know your organs. If you have liver disease, some medications will build up and need lower doses. If you have kidney disease, drugs cleared by the kidneys (like metformin) need dose checks. 4) Watch for signs of too much or too little drug: unusual drowsiness, muscle pain, bleeding, or lack of expected benefit. 5) Consider testing if you have strange reactions. Genetic tests for CYP enzymes are becoming common and can explain why a drug acts differently for you.

Buying meds online? Make sure the pharmacy asks about current prescriptions and can provide counseling. If a site skips that, pause. A safe pharmacy will warn you about interactions and dosing, not just sell pills.

Understanding metabolism helps you get the right dose, avoid surprises, and work with your doctor better. If something feels off after starting a drug, don’t ignore it—ask for a review of how that medicine is metabolized and whether adjustments are needed.