What it means
After oral ingestion, a drug is absorbed from the gastrointestinal tract into the portal circulation — the blood supply that flows directly to the liver before reaching the systemic (general) circulation. In the gut wall and liver, drug-metabolising enzymes — primarily the cytochrome P450 (CYP) family ↗, especially CYP3A4 — biotransform a fraction of the drug into metabolites before it can reach target tissues. This pre-systemic elimination is called the first-pass effect.
The fraction that escapes first-pass metabolism and reaches the systemic circulation is the oral bioavailability (F). It can be expressed as: F = f_a × f_g × f_h, where f_a is the fraction absorbed from the gut lumen, f_g is the fraction surviving gut-wall metabolism, and f_h is the fraction surviving hepatic first-pass extraction. For drugs with high hepatic extraction ratios (ER > 0.7), oral bioavailability can be below 30%.
First-pass metabolism has two important clinical consequences. First, the required oral dose is higher than the IV dose — sometimes by a factor of 3–10×. Second, bioavailability is highly variable between patients because first-pass extraction is sensitive to CYP enzyme inhibitors, inducers, hepatic blood flow, and genetic polymorphisms in CYP2D6 and CYP2C19. The FDA CYP interaction table ↗ classifies drugs by first-pass extraction risk.
Formula
| Equation | Variables | Notes |
|---|---|---|
| F = f_a × f_g × f_h | f_a = fraction absorbed; f_g = gut survival; f_h = hepatic survival | Full oral bioavailability decomposition |
| f_h = 1 − ER | ER = hepatic extraction ratio = CL_H / Q_H | Fraction surviving liver first-pass |
| ER = CL_int × f_u / (Q_H + CL_int × f_u) | CL_int = intrinsic clearance; f_u = unbound fraction; Q_H = hepatic blood flow | Well-stirred liver model |
| Oral dose = IV dose / F | F = oral bioavailability (0–1) | Dose adjustment for first-pass effect |
Hepatic blood flow (Q_H ≈ 1.5 L/min at rest) is the rate-limiting factor for high-extraction drugs. Disease states (cirrhosis, heart failure) or drugs that alter hepatic blood flow directly change first-pass extraction for flow-limited drugs. See StatPearls hepatic clearance ↗.
Example: morphine oral vs. IV
Morphine ↗ has approximately 25% oral bioavailability due to extensive hepatic first-pass metabolism (primarily by UGT2B7 to morphine-6-glucuronide and morphine-3-glucuronide). A 10 mg IV dose achieves equivalent systemic exposure to approximately 30 mg oral:
| Route | Dose | F | Systemic exposure | Notes |
|---|---|---|---|---|
| Intravenous | 10 mg | 1.00 | 10 mg | No first-pass |
| Oral | 30 mg | ~0.33 | ~10 mg | Extensive first-pass |
| Sublingual | 10 mg | ~0.40 | ~4 mg | Partial bypass |
The active metabolite morphine-6-glucuronide (M6G) is itself pharmacologically active and renally cleared — in renal failure, M6G accumulates and can cause prolonged opioid effect. See the morphine DailyMed label ↗ and our metabolite glossary page.
Common mix-ups
These terms are often used interchangeably. Strictly, the "first-pass effect" encompasses all pre-systemic elimination including gut-lumen degradation, whereas "first-pass metabolism" refers specifically to enzymatic biotransformation in the gut wall and liver.
A drug can be completely absorbed from the GI tract yet have very low oral bioavailability due to gut-wall or hepatic first-pass metabolism. Bioavailability is determined by both absorption and first-pass extraction — not absorption alone.
Rectal administration only partially bypasses first-pass metabolism. Blood from the upper rectum drains into the portal system (hepatic first-pass). Only the inferior and middle rectal veins drain directly into systemic circulation, bypassing the liver. F for rectal drugs varies widely.
The gut wall contains substantial CYP3A4, contributing up to 50% of total first-pass extraction for drugs like cyclosporine, midazolam, and felodipine. Grapefruit juice inhibits intestinal CYP3A4 but not hepatic CYP3A4, selectively boosting gut-wall F.
FAQ
What is first-pass metabolism?
First-pass metabolism (also called first-pass effect or pre-systemic elimination) is the enzymatic biotransformation of an orally administered drug in the intestinal wall and liver before it enters the systemic circulation. It substantially reduces oral bioavailability compared to IV administration. See the StatPearls pharmacokinetics overview ↗ for a clinical introduction.
Which enzymes are responsible for first-pass metabolism?
CYP3A4 ↗ (the most abundant CYP enzyme in the liver and gut wall) handles the majority of first-pass metabolism. CYP2D6, CYP2C9, and CYP2C19 also contribute. UDP-glucuronosyltransferases (UGTs) and sulfotransferases (SULTs) handle phase II gut-wall conjugation. Relevant CYP enzyme inhibition by co-administered drugs can dramatically increase bioavailability of high-extraction drugs.
Which drugs have high first-pass metabolism?
Drugs with extensive first-pass metabolism include: morphine ↗ (oral F ~25%), lidocaine (oral F ~35%), propranolol (oral F ~25–30%), nitroglycerin (oral F <1%), testosterone, and verapamil (oral F ~20%). These drugs are often given sublingually, transdermally, or intravenously to bypass first-pass extraction. See our bioavailability page for F values.
How does first-pass metabolism affect drug dosing?
For drugs with high first-pass metabolism, oral doses must be much higher than IV doses to achieve equivalent systemic exposure. For morphine, the oral-to-IV dose ratio is approximately 3:1 (F ~0.33). The oral bioavailability (F) directly determines the dose adjustment: oral dose = IV dose / F. Consult individual DailyMed drug labels ↗ for route-specific dosing.
Can drug interactions affect first-pass metabolism?
Yes. CYP inhibitors (e.g. ketoconazole, grapefruit juice ↗ via naringenin/furanocoumarins) can markedly reduce first-pass extraction, dramatically increasing oral bioavailability of high-extraction drugs. CYP inducers (e.g. rifampicin) increase first-pass clearance, reducing oral exposure. See CYP enzyme inhibition and enzyme induction for details.
What is hepatic extraction ratio and how does it relate to first-pass metabolism?
The hepatic extraction ratio (ER) is the fraction of drug removed from portal blood in a single pass through the liver. High-extraction drugs (ER > 0.7, e.g. morphine, lidocaine, propranolol) have extensive first-pass metabolism and highly variable oral bioavailability. Low-extraction drugs (ER < 0.3, e.g. warfarin, diazepam) have minimal first-pass effect. F ≈ (1 − ER_gut) × (1 − ER_liver).
How can first-pass metabolism be bypassed?
Routes that bypass hepatic first-pass metabolism include: sublingual (e.g. nitroglycerin, buprenorphine ↗), transdermal (e.g. fentanyl, nicotine), inhalation (e.g. salbutamol), rectal (partial bypass), and intravenous or intramuscular administration. Prodrugs may be used to improve oral bioavailability by surviving first-pass extraction in an inactive form.