What is first-pass metabolism?

First-pass metabolism, or the first-pass effect, is the metabolism of a drug before it reaches the systemic circulation. After oral dosing, absorbed drug crosses the intestinal wall and travels in the portal vein to the liver, and both the gut wall and the liver can metabolize it on the way. The result is a lower oral bioavailability, so less of each oral dose reaches the rest of the body than after an IV dose.

Which routes of administration avoid first-pass metabolism?

IV, sublingual and buccal, inhaled, intranasal and transdermal routes avoid hepatic first-pass metabolism because the drug enters veins that do not drain into the portal vein. IM and subcutaneous injections also reach the systemic circulation without passing through the liver first. Vaginal dosing largely avoids it, while rectal dosing avoids it only partly. Oral dosing has the largest first-pass effect.

Does rectal administration avoid first-pass metabolism?

Only partly. The upper rectum drains through the superior rectal vein into the inferior mesenteric vein and then the portal system, so drug absorbed there reaches the liver first. The distal rectum drains through the middle and inferior rectal veins into the internal iliac veins, which are systemic. Overall, about half of a rectally absorbed dose goes directly to the liver.

Why is nitroglycerin given sublingually?

Nitroglycerin undergoes more than 90% hepatic clearance in a single pass through the liver, so a swallowed dose would be largely destroyed before it could act. Under the tongue, it is absorbed into veins that bypass the portal vein, and its vasodilator effect starts within about 1 to 3 minutes and peaks by 5 minutes. Transdermal patches also avoid first-pass metabolism and are used for prevention rather than attacks.

How does first-pass metabolism change oral dosing?

Bioavailability (F) is the fraction of a dose that reaches the systemic circulation unchanged, calculated by comparing dose-corrected AUC after oral and IV dosing. To match IV exposure, the oral dose equals the IV dose divided by F, so a drug with F of 0.25 needs four times the IV dose orally. Morphine is the classic example: about 30 mg oral is roughly equivalent to 10 mg parenteral.

How does liver disease affect drugs with high first-pass metabolism?

Liver disease reduces first-pass extraction and slows clearance, so more of each oral dose reaches the circulation and stays longer. In patients with cirrhosis, steady-state propranolol concentrations were about 2.5 times higher and its half-life lengthened, and lidocaine's half-life can double or more with liver dysfunction. These drugs therefore call for caution, and often lower doses, in liver disease.

First-Pass Metabolism: 3 Tips, Bypass Routes & Key Drugs

First-pass metabolism (the first-pass effect) is the loss of an oral drug to metabolism in the gut wall and liver before it reaches the systemic circulation: everything absorbed from the gut travels through the portal vein to the liver first, so the first pass lowers oral bioavailability (F). The classic example is nitroglycerin, which loses more than 90% of a dose in a single pass through the liver and is therefore given sublingually. IV, sublingual or buccal, inhaled, intranasal and transdermal routes avoid the hepatic first pass; rectal dosing avoids it only partly, because about half of the drug absorbed from the rectum still goes straight to the liver.

Tip 1: Follow the blood (gut → portal vein → liver)

The route decides whether the liver gets the first bite. Picture an oral dose: gut lumen → intestinal wall → portal vein → liver → hepatic veins → systemic circulation. Enzymes in the intestinal wall and the liver both metabolize drug on the way through. Any route whose veins drain somewhere other than the portal vein skips that step.

Route	Hepatic first pass?	Why
Oral (PO)	Yes, maximal	Absorbed drug passes the gut wall and then the liver before reaching the rest of the body
Sublingual / buccal	No	Oral-cavity veins drain toward the superior vena cava, bypassing the portal vein
Rectal	Partial	The superior rectal vein drains to the portal system via the inferior mesenteric vein; the distal rectum drains through the middle and inferior rectal veins to the internal iliac veins. About half of the absorbed dose goes straight to the liver
IV	No	Enters the systemic circulation directly; it is the reference for absolute bioavailability
IM / SC	No hepatic first pass	Drug crosses capillary membranes into the systemic circulation. Large proteins (over 20,000 g/mol) are absorbed mainly via lymphatics, where proteolytic enzymes can degrade some of the dose: a first pass that is not hepatic
Transdermal	No	Systemic availability of estradiol from a patch is about 20 times that of oral estradiol, because there is no first pass
Inhaled	No	Drug absorbed in the lungs enters the pulmonary veins and goes straight to the systemic circulation
Intranasal	No	Avoids the first-pass effect
Vaginal	Largely no	Vaginal venous plexuses drain mainly into the internal iliac veins

Exam framing: "rapid effect and no first pass" points to sublingual (or IV). "Avoids first pass but gives steady levels for hours or days" points to transdermal.

Tip 2: Do the bioavailability math

Bioavailability (F) is the fraction of a dose that reaches the systemic circulation unchanged. Absolute bioavailability is measured against an IV reference dose, and the area under the concentration-time curve (AUC) is the most reliable measure of how much drug got in.

F = (AUC oral ÷ oral dose) ÷ (AUC IV ÷ IV dose). IV is the 100% reference.
Oral dose for the same exposure = IV dose ÷ F. With F = 0.25, the oral dose must be 4 times the IV dose.
Morphine shows the gap: its oral bioavailability is about 20-40%, because pre-systemic elimination in the gut wall and liver lets only about 40% of a dose reach the central compartment. Merck's equianalgesic table pairs 10 mg parenteral morphine with 30 mg oral.
Low F does not always mean first pass. Poor solubility or too little time for absorption can lower F, and food, GI disease or previous GI surgery can change it. First pass is the answer when absorption is good but F is still low: propranolol is almost completely absorbed, yet only about 25% reaches the systemic circulation.

Hepatic extraction. Drugs that the liver strips efficiently in one pass (nitroglycerin clears more than 90% in one pass) depend heavily on hepatic blood flow and liver function:

Less liver blood flow, higher levels: beta-blockers raise lidocaine levels by reducing hepatic blood flow and lidocaine clearance.
Liver disease, higher exposure: in cirrhosis, steady-state propranolol concentrations rose 2.5-fold and its half-life lengthened from 2.9 to 7.2 hours; lidocaine's half-life can double or more with liver dysfunction. Portal hypertension anatomy is covered in the cirrhosis and portal hypertension guide.
Gut-wall enzymes count too: grapefruit juice blocks intestinal CYP3A4, so more of an oral CYP3A4 substrate gets into the blood. The full inducer and inhibitor lists are in the CYP450 guide.

For more F and AUC drills, see the bioavailability mnemonic.

Tip 3: Know the high-first-pass drugs and their workaround routes

Drug	First-pass fact	Workaround	Exam pearl
Nitroglycerin	More than 90% cleared in one pass through the liver	Sublingual tablet or spray for attacks; transdermal patch for prevention	Sublingual relief starts in about 1-3 minutes; never combine with PDE-5 inhibitors
Lidocaine	High first-pass metabolism	IV for ventricular arrhythmias	Mexiletine is structurally similar but orally active, because its first pass is low
Propranolol	Almost fully absorbed; only about 25% reaches the circulation	Oral doses allow for the loss	Cirrhosis raises steady-state levels 2.5-fold
Morphine	Oral F about 20-40%	Oral dose about 3 times the parenteral dose	10 mg parenteral ≈ 30 mg oral
Estradiol	Oral estradiol undergoes first-pass metabolism	Transdermal patch	Patch systemic availability is about 20 times oral

Nitroglycerin details that get tested:

Dose for an attack: one sublingual tablet at the first sign of angina, repeated every 5 minutes if needed, no more than 3 tablets in 15 minutes. The vasodilator effect starts about 1-3 minutes after a dose and peaks by 5 minutes.
Sublingual is not 100%: the absolute bioavailability of sublingual nitroglycerin tablets is about 40%, and variable. Bypassing the liver is not the same as perfect absorption.
Its half-life is only minutes (mean 2-3 minutes), so a tablet treats an attack, while prevention needs a long-acting form such as a patch. See stable vs unstable angina for where nitrates fit.

Lidocaine and mexiletine are both class IB sodium channel blockers; the rest of the class system is in the antiarrhythmics question.

How USMLE tests it

"Well absorbed but low oral bioavailability" → first-pass metabolism (not poor absorption).
"Fastest relief of an angina attack" → sublingual nitroglycerin, which bypasses the portal vein.
"Oral drug similar to lidocaine" → mexiletine.
"Patient with cirrhosis has an exaggerated response to a usual oral dose" → less first-pass extraction and slower clearance, so more drug reaches the circulation (propranolol is the classic).
"Rectal suppository avoids first pass?" → only partly: the upper rectum drains to the portal system, the lower rectum to the systemic veins.
"Grapefruit juice raises levels of an oral drug" → gut-wall CYP3A4 inhibition: a first-pass interaction, not a renal one.
Calculation: oral dose = IV dose ÷ F.

Practice question

In a study, a new drug is given to healthy volunteers. A 10-mg IV dose produces an AUC of 40 mg·h/L. On another day, a 40-mg oral dose of the same drug produces an AUC of 40 mg·h/L. Studies with a radiolabeled dose show that more than 95% of the oral dose is absorbed from the gut. Which of the following best describes this drug's oral bioavailability?

A. 25%, because of extensive first-pass metabolism in the gut wall and liver B. 25%, because of poor absorption across the intestinal mucosa C. 25%, because the drug has a large volume of distribution D. 100%, because the oral and IV doses produced the same AUC E. 400%, because the oral dose was 4 times the IV dose

Answer: A. F = (AUC oral ÷ oral dose) ÷ (AUC IV ÷ IV dose) = (40 ÷ 40) ÷ (40 ÷ 10) = 1 ÷ 4 = 25%. Because nearly all of the dose is absorbed, the missing 75% was lost after absorption but before the systemic circulation: that is first-pass metabolism.

B: the radiolabel data show near-complete absorption, so poor absorption cannot explain the low F.
C: volume of distribution changes plasma concentrations and half-life, not the fraction of the dose that reaches the circulation.
D: equal AUCs after unequal doses mean F is below 100%; AUC must be corrected for dose.
E: F can never exceed 100%; a bigger oral dose was needed precisely because three-quarters of it is lost.

Key takeaways

First pass = presystemic loss in the gut wall and liver after oral dosing, because the portal vein delivers absorbed drug to the liver first.
No hepatic first pass: IV, sublingual/buccal, inhaled, intranasal, transdermal, IM and SC. Rectal is partial (about half of the absorbed drug goes straight to the liver).
F = dose-corrected AUC oral ÷ AUC IV; oral dose = IV dose ÷ F.
Classic drugs: nitroglycerin (more than 90% hepatic extraction: sublingual or patch), lidocaine (IV; mexiletine is the oral analog), propranolol (about 25% reaches the circulation), morphine (oral F about 20-40%; 10 mg parenteral ≈ 30 mg oral).
Liver disease and low hepatic blood flow raise levels of high-extraction drugs; gut-wall CYP3A4 inhibition by grapefruit raises levels of oral CYP3A4 substrates.