How the Body Processes Herbs and Supplements: Liver Enzymes and Transporters Explained
A plain-language look at how enzymes and transporters handle medicines, and how some herbs and foods can speed up or slow down that process, using FDA and NIH sources.
Content last updated . This page has not been medically reviewed.
Quick answer
Your body breaks down many medicines with proteins called enzymes, and moves some into and out of cells with proteins called transporters. Some herbs, supplements and foods can change how those proteins work. When that happens a medicine can build up (more side effects) or clear faster (less effect). MedlinePlus puts it simply: some supplements can affect the way your body processes medications. [1]
This page explains the idea. It does not tell you what to do about a specific medicine; your pharmacist or prescriber can.
Key terms
- Metabolism: the body's process of breaking a substance down, for example turning a medicine into a form that is less active or easier to remove.
- Enzyme: a protein that speeds up a chemical step. The FDA calls one important example, CYP3A4, "a vital enzyme" that helps break down many drugs. [2]
- Cytochrome P450 (CYP): a family of enzymes. The FDA's drug-interaction tables use the abbreviation CYP for cytochrome P450 and number the members, for example CYP3A4/5. [3]
- Transporter: a protein that helps move a drug into or out of cells, such as in the small intestine. The FDA says some drugs are moved into the body's cells by transporters. [2]
- Induction and inhibition: making an enzyme or transporter more active (induction) or less active (inhibition). We use these two words below because the sources do.
Where this happens
The FDA says that many drugs are broken down with the help of CYP3A4 in the small intestine. [2] The sources we read also mention the liver: NCCIH reports interactions between chamomile and "some drugs metabolized by the liver". [4] We did not read a source here that lays out, enzyme by enzyme, where each one works in the body, so we do not give that detail.Two ways an interaction can go
| Direction | What the source says | Example |
|---|---|---|
| Medicine builds up | The FDA says that when grapefruit juice blocks intestinal CYP3A4, more of the drug enters the blood and stays in the body longer. [2] | Grapefruit juice with certain medicines. See Grapefruit and medicine interactions. |
| Medicine clears faster or less gets in | MedlinePlus says certain supplements can cause medications to break down faster than normal, making them less effective. [1] The FDA says grapefruit juice can affect transporters so less of some drugs, such as fexofenadine, enters the blood. [2] | St. John's wort, below |
The same MedlinePlus article says other supplements may slow the process, causing medications to stay in the body too long and raising the risk of side effects. [1]
The best-documented herb example
NCCIH's clinician digest says St. John's wort (Hypericum perforatum; accepted name per GBIF [5]) has been found to have an overall high risk of drug interaction because it is a potent inducer of both cytochrome P-450 enzymes and intestinal P-glycoprotein. [4] NCCIH states that clinically significant interactions with medicines including cyclosporine, oral contraceptives, warfarin (coumadin), digoxin and benzodiazepines have been documented. [4] MedlinePlus says the herb can keep medicines such as birth control pills, antidepressants, blood thinners and some cancer treatments from working correctly. [1] Read more in St. John's wort interactions.Why one person can differ from another
The FDA notes that the amount of the CYP3A4 enzyme in the intestine varies from person to person, so grapefruit juice may affect people differently even when they take the same drug. [2] It adds that the severity can depend on the person, the drug and the amount of juice. [2] Proper evidence not found: in the sources we read we found no plain-language statement on how genes, age or illness change herb-drug effects, so we do not state that here.How firm is this?
Be careful with the word "enzyme" in marketing. A herb that changes an enzyme in a test tube or in animals has not necessarily changed how a medicine behaves in people. NCCIH says most identified herb-drug interactions are hypothetical, inferred from animal studies or cellular assays, or from other indirect means. [4] It also says that herbs can have the same dangers as other pharmacologically active compounds, and that it is a daunting challenge to identify all interactions of concern. [4] See Documented, reported and theoretical risks.The product matters too. NCCIH cites a finding that green tea at high doses reduced blood levels of the drug nadolol. [4] That is a result for a high amount of green tea and one drug, not a statement about every tea.
What to do with this information
- Tell your pharmacist and prescriber about every herb, supplement and food you use regularly. NCCIH suggests bringing a written list or the bottles. [6]
- For medicines where small changes matter (NCCIH lists carbamazepine, cyclosporine, digoxin, phenytoin and warfarin), ask before adding anything. [7]
- Do not stop or change a prescribed medicine yourself; ask the prescriber or pharmacist.
- For more by medicine group, see Interactions by drug class and How interactions work.
- Product quality also affects what you actually take; see Quality testing and certifications.
Frequently asked questions
What is CYP3A4?
It is an enzyme that the FDA says helps break down many drugs in the small intestine. [2]
Can a supplement make a medicine weaker?
Yes, some can. MedlinePlus says certain supplements can cause medications to break down faster than normal, making them less effective. [1]
Can a supplement make a medicine stronger?
Yes, some can. NCCIH's Know the Science module says taking a medication and a supplement together may increase the medication's effects and raise the risk of side effects. [8]
Does every herb affect liver enzymes?
Proper evidence not found in the sources we read for a general statement like that. NCCIH describes specific herbs one by one, and says many concerns are still hypothetical. [4]
Why does my pharmacist ask about grapefruit?
Because the FDA says grapefruit can affect how some medicines work, and the effect varies by person, drug and amount. [2]
References
- NIH MedlinePlus Magazine. Did you know? Supplements and medications can interact in unexpected ways. National Library of Medicine, US, updated January 2025. Read 2026-10-23.
- US Food and Drug Administration. Grapefruit Juice and Some Drugs Don't Mix. Content current as of 2021-07-01. Read 2026-10-23.
- US Food and Drug Administration. Drug Development and Drug Interactions: Table of Substrates, Inhibitors and Inducers. Read 2026-10-23.
- NCCIH. Herb-Drug Interactions (Clinical Digest). National Institutes of Health, US, December 2024. Read 2026-10-23.
- GBIF Secretariat. GBIF Backbone Taxonomy: Hypericum perforatum L.. Read 2026-10-23.
- NCCIH. Know the Science: Talk With Your Health Care Providers. National Institutes of Health, US. Read 2026-10-23.
- NCCIH. Know the Science: Some Medication-Supplement Interactions Can Be Serious. National Institutes of Health, US. Read 2026-10-23.
- NCCIH. Know the Science: How Medications and Supplements Can Interact, "Some Supplements May Increase the Effects". National Institutes of Health, US. Read 2026-10-23.
Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.