What is bioavailability in simple terms?

Bioavailability is the proportion of a dose that reaches your systemic bloodstream in an active, unchanged form. Intravenous administration is the reference point at 100%; every other route — oral, sublingual, inhaled, transdermal — is measured against that. Oral bioavailability is almost always lower because of barriers in the gut and liver before the compound reaches general circulation.

Why does the oral route have lower bioavailability than inhalation or injection?

Oral administration requires a compound to survive stomach acid and digestive enzymes, cross the gut wall, travel through the portal vein, and pass through the liver — all before reaching systemic circulation. The liver in particular metabolises a significant fraction of many compounds in this single pass. Inhalation and injection routes bypass this hepatic first-pass step, so a higher proportion of the dose typically reaches the bloodstream intact.

Is the first-pass effect the same for everyone?

No. Hepatic enzyme activity — particularly the cytochrome P450 family — varies between individuals due to genetics, other medicines, diet, and health status. This means two people taking an identical oral dose can end up with quite different plasma concentrations of the active compound. It's one reason pharmacokinetic data shows wide inter-individual variability.

Why can't most peptides be taken orally?

Peptides are chains of amino acids linked by peptide bonds, and those bonds are targets for protease enzymes throughout the gastrointestinal tract. Most peptides are broken down into their component amino acids — or smaller, inactive fragments — before they can be absorbed intact. That's why injectable routes, including subcutaneous injection, are the standard delivery method for peptide-based medicines that need to reach systemic circulation in an active form.

How do researchers measure bioavailability in practice?

The standard method compares the area under the plasma concentration-time curve (AUC) after the route being tested against the AUC after an intravenous dose of the same compound in matched subjects. The ratio of the two gives the bioavailability fraction (F). When IV administration isn't practical, researchers sometimes use 'relative bioavailability' — comparing two non-IV formulations against each other — which answers whether one formulation delivers more drug than another, rather than the absolute fraction reaching circulation.

Bioavailability and the First-Pass Effect

By Priya Nandakumar · 21 April 2026 · 7 min read

clear glass measuring cup on white table

Roughly 40–80% of an oral dose of many lipophilic compounds is metabolised before it ever reaches systemic circulation. That number sits in the back of my mind whenever I'm reading a study that only measured plasma concentrations from an intravenous route, then drew conclusions about what an oral dose would do. It's a gap that matters , a lot.

This piece covers the mechanics of bioavailability and the first-pass effect: what they are, how they're measured, and why the route you take a substance determines so much about how the body actually encounters it.

What bioavailability actually means

Bioavailability (abbreviated F, for "fraction") is the proportion of an administered dose that reaches systemic circulation in an unchanged, active form. Intravenous delivery is the reference point , it's defined as 100% bioavailable, because the dose goes straight into the bloodstream with nothing in the way. Every other route is measured against that benchmark.

Oral bioavailability is almost always lower, sometimes dramatically so. The substance has to survive the gastrointestinal environment, cross the gut wall, pass through the portal circulation, and then run the gauntlet of hepatic metabolism , all before a single molecule touches the general bloodstream. That full sequence is what pharmacologists call the first-pass effect.

It's worth understanding that bioavailability isn't a fixed property of a molecule alone. It's a property of a molecule and a formulation and a delivery route and an individual's physiology. Two people taking the same oral dose of the same compound can have meaningfully different plasma concentrations, depending on gut motility, enzyme expression, body composition, and what they ate beforehand.

The first-pass effect: a two-stage filter

The first pass has two major sites of action.

The first is the gut wall itself. Enterocytes; the absorptive cells lining the small intestine, express metabolising enzymes, most notably members of the cytochrome P450 family (particularly CYP3A4) and efflux transporters like P-glycoprotein. P-gp actively pumps absorbed molecules back into the intestinal lumen, reducing net absorption. CYP3A4 begins chemically modifying them. So some fraction of the dose never makes it past this first barrier.

Whatever does cross the gut wall then enters the portal vein and travels directly to the liver, not to the heart, not to the lungs, not to the rest of the body. The liver. Hepatic enzymes (again, largely the CYP450 superfamily) metabolise this portal blood before it enters the inferior vena cava and reaches systemic circulation. This is the hepatic first pass. For compounds with high hepatic extraction ratios, the liver can remove 70, 80, even 90% of the absorbed dose in a single pass.

The combined result? A compound might be well absorbed across the gut wall but still have poor oral bioavailability because the liver clears so much of it. Understanding this distinction; absorption versus bioavailability, is fundamental to reading pharmacokinetics data correctly.

How lipophilicity fits in

Most cannabinoids are highly lipophilic. Cannabidiol, tetrahydrocannabinol, and related compounds dissolve readily in fat but poorly in water. That creates a somewhat counterintuitive situation: lipophilic compounds are generally well absorbed across lipid-rich cell membranes, but they also tend to be extensively metabolised by hepatic enzymes, and they accumulate in fatty tissue, which creates a depot effect that extends their presence in the body long after plasma concentrations have dropped.

Published estimates for oral THC bioavailability in humans sit roughly between 4% and 20%, with high inter-individual variability. CBD oral bioavailability estimates are similarly wide; some reviews cite figures as low as 6% under fasted conditions. These numbers come from studies with small sample sizes and varying formulations, so they're indicative rather than definitive. But the range alone tells you something: oral delivery is inherently inconsistent for these molecules without deliberate formulation strategies to improve it.

Those strategies, lipid-based delivery systems, nanoemulsions, self-emulsifying drug delivery, exist precisely to address first-pass losses. They increase solubilisation in the GI tract and, in some cases, promote lymphatic absorption, which bypasses the portal vein entirely and sidesteps hepatic first-pass metabolism. I'd argue this area of formulation science is underappreciated in public-facing discussions about plant-based medicines; the form a compound takes matters as much as the compound itself.

Routes that sidestep the first pass

Inhalation, sublingual, and intravenous routes all largely avoid hepatic first-pass metabolism, which is why they generally produce faster onset and higher bioavailability than oral routes for the same compound.

Inhalation delivers compounds directly to the pulmonary circulation, from which they enter systemic blood rapidly. Sublingual absorption uses the highly vascularised mucosa under the tongue; absorbed molecules enter the systemic circulation via the sublingual and facial veins, not the portal system. Transdermal delivery is slower but also avoids first-pass metabolism; though the skin is its own significant barrier, particularly for polar or high-molecular-weight compounds.

For peptides, this is especially relevant. Peptide bonds are cleaved readily by proteases in the GI tract, meaning most peptides have negligible oral bioavailability unless specifically engineered otherwise. That's why injectable routes, including subcutaneous injection, are standard for peptide-based medicines. Subcutaneous injection bypasses both GI degradation and hepatic first-pass, allowing much of the intact molecule to reach systemic circulation. The absorption is slower than intravenous (because the compound has to diffuse from the injection site through interstitial fluid and into capillaries), but the bioavailability is generally far higher than oral for these molecules. I find this distinction endlessly interesting; the same chemical logic that makes a peptide difficult to deliver orally is what makes it such a poor fit for over-the-counter formats.

Measuring bioavailability: AUC and the reference standard

Bioavailability is calculated by comparing the area under the plasma concentration-time curve (AUC) following the route of interest against the AUC following intravenous administration of the same dose. The ratio gives you F. Simple in principle; messier in practice, because you need matched subjects, controlled conditions, and reliable assays, none of which are cheap or easy.

Relative bioavailability is sometimes reported instead: comparing two non-IV formulations against each other rather than against an IV reference. This is common in drug development when IV administration isn't practical or ethical. It answers the question "is formulation A better than formulation B?" rather than "what fraction of the dose actually reaches circulation?"

I spent an afternoon last month going back through the Huestis 2007 review in Chemistry & Biodiversity on cannabinoid pharmacokinetics, still one of the more thorough treatments of this specific topic. The variability data in that paper is humbling, honestly. It makes a strong case for why single-study bioavailability estimates for cannabinoids should be treated as rough guides, not settled numbers.

Why this matters for reading research

A lot of preclinical cannabinoid research uses intraperitoneal injection in rodents; a route with very different absorption characteristics to oral human dosing. When those results get translated into commentary about human responses to oral products, the pharmacokinetic gap is usually glossed over. It shouldn't be.

Similarly, studies comparing full-spectrum preparations to isolates need to control carefully for formulation differences that affect bioavailability, not just for the presence or absence of minor cannabinoids and terpenes. If one formulation is a lipid-based emulsion and the other is a crystalline powder suspended in a carrier, you're comparing delivery systems as much as you're comparing chemical composition. Keeping those variables disentangled is genuinely hard, which is one reason the entourage effect literature is still so difficult to interpret cleanly.

The short version is this: bioavailability and first-pass metabolism aren't footnotes to pharmacology, they're load-bearing concepts. Every time a route of administration, a formulation type, or a plasma-concentration curve appears in a study, the first-pass effect is somewhere in the background, shaping the numbers. Getting comfortable with that underlying machinery makes you a much sharper reader of the research.

Sources

, Priya Nandakumar, Pharmacology writer; peptides & PK

Common questions

What is bioavailability in simple terms?: Bioavailability is the proportion of a dose that reaches your systemic bloodstream in an active, unchanged form. Intravenous administration is the reference point at 100%; every other route — oral, sublingual, inhaled, transdermal — is measured against that. Oral bioavailability is almost always lower because of barriers in the gut and liver before the compound reaches general circulation.
Why does the oral route have lower bioavailability than inhalation or injection?: Oral administration requires a compound to survive stomach acid and digestive enzymes, cross the gut wall, travel through the portal vein, and pass through the liver — all before reaching systemic circulation. The liver in particular metabolises a significant fraction of many compounds in this single pass. Inhalation and injection routes bypass this hepatic first-pass step, so a higher proportion of the dose typically reaches the bloodstream intact.
Is the first-pass effect the same for everyone?: No. Hepatic enzyme activity — particularly the cytochrome P450 family — varies between individuals due to genetics, other medicines, diet, and health status. This means two people taking an identical oral dose can end up with quite different plasma concentrations of the active compound. It's one reason pharmacokinetic data shows wide inter-individual variability.
Why can't most peptides be taken orally?: Peptides are chains of amino acids linked by peptide bonds, and those bonds are targets for protease enzymes throughout the gastrointestinal tract. Most peptides are broken down into their component amino acids — or smaller, inactive fragments — before they can be absorbed intact. That's why injectable routes, including subcutaneous injection, are the standard delivery method for peptide-based medicines that need to reach systemic circulation in an active form.
How do researchers measure bioavailability in practice?: The standard method compares the area under the plasma concentration-time curve (AUC) after the route being tested against the AUC after an intravenous dose of the same compound in matched subjects. The ratio of the two gives the bioavailability fraction (F). When IV administration isn't practical, researchers sometimes use 'relative bioavailability' — comparing two non-IV formulations against each other — which answers whether one formulation delivers more drug than another, rather than the absolute fraction reaching circulation.