Category: Science Explained
Cytochrome P450 is not the only reason drugs interact. P-glycoprotein is a transporter that actively pumps compounds out of cells, and inducing or blocking it can change how much of a medicine reaches your bloodstream.
Category: Science Decoded | Reading time: ~8 min | Level: Intermediate
Most explanations of drug interactions stop at the liver, at enzymes chopping medicines into pieces the body can clear. That is a big part of the story, but it is not all of it, and the missing part explains some interactions that otherwise look impossible. Sometimes a herb lowers the level of a drug whose metabolism it never touches. How?
The answer is a protein called P-glycoprotein, a tiny pump built into the walls of certain cells whose job is to throw compounds back out. It is easy to overlook because it does nothing dramatic and has an unmemorable name, but it quietly decides how much of some medicines you absorb and whether they can reach your brain. This is a plain guide to what P-glycoprotein is, where it sits, why it matters for interactions, and why St John's wort keeps appearing in these stories.
P-glycoprotein, often shortened to P-gp, is a transporter protein embedded in the membranes of certain cells that actively pumps many drugs and other compounds back out [1]. It does not change a drug chemically. It physically moves it, ejecting substances it recognises before or after they enter a cell [1][3].
The idea in one sentence: P-glycoprotein is a bouncer at the cell door, reducing how much of a compound stays inside or crosses a barrier [3]. It is a normal, protective system, one of the body's ways of keeping unwanted substances out of sensitive tissues. But because it controls how much of a medicine gets in and stays in, it also shapes drug behaviour, and changes in how hard it pumps become a source of interactions entirely separate from how a drug is broken down.
It helps to place P-glycoprotein next to the more famous cytochrome P450 system, because they are often confused and they do genuinely different jobs.
Cytochrome P450 is a family of enzymes that chemically break drugs down so the body can eliminate them. That is metabolism. P-glycoprotein transports compounds out of cells. That is movement [1][3]. One dismantles, the other relocates. They frequently operate in the same tissues, such as the gut wall and the liver, and some substances affect both, but the mechanisms are distinct. This distinction is the whole reason P-glycoprotein deserves its own page: a drug can interact through the transporter even when its metabolism is completely untouched, which is why modern interaction checking looks at transporters as well as enzymes.
Location is everything with P-glycoprotein, because it is concentrated at the body's gateways and barriers [1][3].
In the lining of the intestine, it pumps compounds back into the gut, limiting how much of a drug is absorbed in the first place. In the liver and kidneys, it helps move compounds toward elimination. And in the blood-brain barrier, the system that guards what enters the brain, it pumps many substances back out to keep them from reaching brain tissue [3].
Because it sits at these strategic points, its activity influences three things at once: how much of a medicine is absorbed, how quickly it is cleared, and whether it can reach the central nervous system. Turn the pump up or down at the gut wall and you change absorption; do the same at the blood-brain barrier and you change how much of a drug reaches the brain. Its positions are precisely why a change in P-glycoprotein activity can matter clinically.
P-glycoprotein interactions follow the same two directions as enzyme ones, which makes them easy to reason about once you know the system exists [3].
If something induces P-glycoprotein, it pumps more vigorously. Less of an affected drug is absorbed or retained, blood levels fall, and the drug can become less effective. If something inhibits P-glycoprotein, the pump slows. More drug is absorbed or retained, levels rise, and effects or side effects can increase [3]. Because the transporter handles a wide range of medicines, a substance that changes its activity can move drug levels up or down without ever affecting how the drug is metabolised.
This is the mechanism that closes the earlier puzzle. A herb can lower a drug's level purely by revving up the transporter that pumps it out, no enzyme involvement required.
If one substance illustrates P-glycoprotein interactions, it is St John's wort. It is known to induce P-glycoprotein, and it also induces the major drug-metabolising enzyme CYP3A4 [2]. Hitting both a transporter and a major enzyme is part of why it can lower the blood levels of such a broad range of medicines, and why it is one of the most significant herbal interaction risks documented [2].
Two features are worth holding on to. The effect builds over days to weeks, as induction does, rather than appearing instantly. And because it works through two systems at once, its impact on drug levels can be larger and broader than a single mechanism would produce. This is a well-characterised interaction described in the pharmacology literature, and it is the reason St John's wort carries such prominent warnings for anyone on prescription medicines. For most other botanicals, any P-glycoprotein effect at realistic doses is weak or unproven.
You do not need to track which drugs are P-glycoprotein substrates. You need to know the system exists, so that an interaction without an obvious metabolic cause does not seem mysterious or get dismissed.
If you take prescription medicines, treat a new supplement as a question for a pharmacist or prescriber, exactly as you would for enzyme interactions. Transporter effects are invisible until a medicine behaves differently, and a professional can check the specific combination against proper references. Take St John's wort warnings seriously in particular, because its induction of both P-glycoprotein and CYP3A4 makes it a broad-spectrum interactor. And remember that for medicines with a narrow safety margin, even a modest change in levels can matter, which is a judgement best left to someone who knows your full picture.
P-glycoprotein is not a reason to fear supplements generally. It is a reason to check combinations when you take medicines, because the transporter is a genuine and often overlooked route for interactions. The clearest herbal caution is St John's wort, which should not be combined with prescription medicines without professional advice [2].
For other supplements, a P-glycoprotein effect is usually weak or unproven, but you cannot tell from a label, so a pharmacist or prescriber is the right person to weigh a specific pair.
Pregnant, breastfeeding, or on medication? Check with a healthcare professional first.
When we describe how a herb can affect a medicine, we try to name the real mechanism rather than gesture vaguely at interactions. P-glycoprotein is one of those mechanisms, and St John's wort is its clearest herbal example, so we say so plainly.
Our Remedy Library and Remy explain what botanicals do in the body, but the interaction check for your specific medicine belongs with a pharmacist or prescriber, and we point you there rather than implying our tools replace that step. For the two mechanisms that drive most herb and drug interactions, our companion pieces on St John's wort interactions and on cytochrome P450 fit alongside this one.
1. U.S. Food and Drug Administration (2020). Drug Development and Drug Interactions: Table of Substrates, Inhibitors and Inducers. Regulatory guidance describing P-glycoprotein and other transporters as substrates, inhibitors and inducers relevant to drug disposition. 2. National Center for Complementary and Integrative Health (2023). St. John's Wort. Government evidence synthesis noting induction of drug-metabolising enzymes and transporters and interactions with many medicines. 3. Lund M, Petersen TS, Dalhoff KP (2017). Clinical Implications of P-Glycoprotein Modulation in Drug-Drug Interactions. Drugs. Review of P-glycoprotein's role in absorption and disposition and its interaction potential.
It is a transporter protein embedded in the membranes of certain cells that pumps many drugs and other compounds back out, before or after they get in. Think of it as a bouncer at the cell door, ejecting substances it recognises. Because it reduces how much of a compound stays inside a cell or crosses a barrier, it helps control how much of a medicine you actually absorb and where it can go. It is a normal protective system, but it also shapes drug behaviour and is a source of interactions that have nothing to do with how a drug is broken down.
They do different jobs. Cytochrome P450 is a family of enzymes that chemically break drugs down so the body can clear them. P-glycoprotein does not break anything down; it physically transports compounds out of cells. One is metabolism, the other is movement. They often work in the same tissues, such as the gut and liver, and some substances affect both, but the mechanisms are separate. This is why a drug can interact through P-glycoprotein even if its metabolism is untouched, and why interaction checks consider transporters as well as enzymes.
It is concentrated in tissues that act as barriers or gateways. In the lining of the intestine it pumps compounds back into the gut, limiting absorption. In the liver and kidneys it helps move compounds toward elimination. And in the blood-brain barrier it restricts what can enter the brain. Because it sits at these strategic points, changes in its activity ripple through how much of a medicine is absorbed, how quickly it is cleared, and whether it can reach the central nervous system. Its locations are the reason it matters so much for drug levels.
Through the same two directions as enzymes: induction and inhibition. If something induces P-glycoprotein, it pumps more vigorously, so less of an affected drug is absorbed or retained and blood levels fall, which can make the drug less effective. If something inhibits it, the pump slows, more drug is absorbed or retained, and levels rise, which can increase effects or side effects. Because P-glycoprotein handles many medicines, a substance that changes its activity can affect drug levels without ever touching the drug's metabolism.
Yes. St John's wort is known to induce P-glycoprotein as well as the drug-metabolising enzyme CYP3A4. Inducing both a transporter and a major enzyme is part of why it can lower the blood levels of such a wide range of medicines and why it is one of the most significant herbal interaction risks. The effect builds over days to weeks. This is a well-documented interaction described in the pharmacology literature, and it is the main reason St John's wort carries prominent warnings for people on prescription medicines.
Some compounds have been studied for effects on P-glycoprotein, with results ranging from meaningful to weak or uncertain, and many supplements have little reliable human data either way. St John's wort is the standout for clear induction. For most botanicals, any P-glycoprotein effect at realistic doses is unproven or minor. The practical implication is the same as for enzyme interactions: if you take prescription medicines, a new supplement is worth checking with a pharmacist, because transporter interactions are invisible until a drug behaves differently.
Because it is a major component of the blood-brain barrier, the system that controls which substances can enter the brain from the bloodstream. P-glycoprotein pumps many compounds back out, keeping them from reaching brain tissue. This protects the brain, but it also means that changes in P-glycoprotein activity can alter how much of certain medicines reach the central nervous system. For drugs that act on the brain, or for ones meant to be kept out of it, the transporter's behaviour can be an important and easily overlooked factor.
Very likely through P-glycoprotein. Not every St John's wort interaction runs through the enzyme system; the herb also induces the P-glycoprotein transporter, which handles many drugs independently of how they are metabolised. So a medicine that is a P-glycoprotein substrate can have its levels lowered by St John's wort even if its enzymatic breakdown is unaffected. This is exactly why interaction references consider transporters as well as enzymes, and why a pharmacist is the right person to check a specific pair.
It depends on the medicine and the strength of the effect, which is a judgement for a professional. A weak or uncertain transporter effect may be clinically unimportant for many drugs, while for a medicine with a narrow safety margin even a modest change in levels can matter. Because you cannot tell from a label, the safe approach if you take prescription medicines is to have a pharmacist or prescriber weigh the specific combination rather than assume a natural product cannot affect a transporter.
Yes, and St John's wort is the classic example: it induces both the CYP3A4 enzyme and the P-glycoprotein transporter. When a substance hits both systems, its effect on drug levels can be larger and broader than either mechanism alone, because a medicine may be both metabolised faster and pumped out more. This combined action is part of why some interactions are so pronounced, and it is a reason interaction checking looks at the whole picture rather than a single pathway.