Why can't most peptides simply be taken as a tablet or capsule?

Peptides are broken down rapidly by digestive enzymes in the stomach and small intestine, and the intestinal wall physically blocks their absorption due to their size and hydrophilicity. Oral bioavailability for unmodified peptides is typically below 2%, which makes oral administration impractical for most molecules without significant structural modification or specialised delivery technology.

What does 'half-life' mean for a peptide, and why does it matter clinically?

Half-life is the time taken for the plasma concentration of a substance to fall by 50%. For peptides, a very short half-life (sometimes under two minutes for unmodified endogenous peptides) means the molecule is cleared too quickly to maintain any sustained biological effect. Chemical modifications — such as PEGylation or albumin binding — can extend half-life from minutes to days or weeks, which has significant implications for how often a medicine needs to be administered.

Are peptides metabolised by the same liver enzymes as conventional drugs?

Not primarily. Conventional small-molecule drugs are largely metabolised by cytochrome P450 (CYP) enzymes in the liver. Peptides, by contrast, are cleaved by proteases found throughout the body — in the gut lumen, the bloodstream, the kidneys, and the liver. The metabolic products are amino acids, which re-enter the body's normal amino acid pool. This is one reason peptide metabolites are generally less toxic than those of synthetic small molecules, though each molecule still requires individual safety evaluation.

What is the TGA's position on unapproved peptides in Australia?

The TGA enforces the Therapeutic Goods Act 1989, which requires that medicines — including peptides — be assessed and listed or registered on the Australian Register of Therapeutic Goods (ARTG) before supply. Supplying unapproved peptides in Australia is illegal. The Special Access Scheme and Authorised Prescriber frameworks allow access to unapproved substances in specific, clinician-supervised contexts, but these are regulated pathways for individual patient use, not general supply channels.

How does subcutaneous injection improve peptide bioavailability compared to oral delivery?

Subcutaneous injection deposits the peptide directly into connective tissue beneath the skin, bypassing the gastrointestinal tract entirely. The peptide is absorbed into local capillaries and lymphatics without facing luminal proteases or the intestinal epithelial barrier. Bioavailability via the subcutaneous route typically ranges from 70% to close to 100% for most peptides, compared to well under 2% for oral delivery of the same unmodified molecule.

Peptide Pharmacokinetics

By Priya Nandakumar · 26 June 2026 · 7 min read

a close up of a blue and red pattern on a black background

Why peptides don't play by the usual pharmacokinetic rules

Roughly 95% of an orally swallowed peptide is destroyed before it ever reaches systemic circulation. That number stopped me the first time I properly understood it , I'd been plodding through a long Saturday run near the Cooks River and had the podcast off for once, just turning the figure over in my head. It's not a rounding error or a worst-case scenario. It's the baseline, and it's the reason almost everything interesting about peptide pharmacokinetics starts with the question: how do you get around the gut?

This piece is about the underlying science , absorption, distribution, metabolism, and elimination (ADME) as they apply specifically to peptides. No dosing, no product names, no sourcing. Just the molecules doing their thing.

What a peptide actually is, structurally speaking

A peptide is a short chain of amino acids linked end-to-end via peptide bonds , covalent connections between the carboxyl group of one residue and the amino group of the next, with water released in the process. Most researchers draw the line between "peptide" and "protein" somewhere around 50 amino acid residues, though that boundary is more convention than hard biochemistry.

What that structure means pharmacokinetically is this: peptides are hydrophilic, relatively large compared to classical small-molecule drugs, and they carry a backbone that is chemically identical to the bonds that digestive proteases exist specifically to cleave. The gut is not hostile to peptides by accident; it is exquisitely designed to break them apart. That's its job.

Absorption: the oral bioavailability problem

Bioavailability is the fraction of an administered dose that reaches systemic circulation unchanged. For most oral small molecules, even accounting for first-pass metabolism in the liver, bioavailability might sit anywhere from 30% to over 90%. For unmodified peptides taken orally, it is typically below 2%, and often far lower.

Three mechanisms drive this. First, luminal proteases, enzymes including pepsin in the stomach, trypsin and chymotrypsin in the small intestine, attack peptide bonds directly. Second, the intestinal epithelium presents a physical barrier: the tight junctions between enterocytes restrict paracellular passage of anything above roughly 500–700 Daltons molecular weight, and peptides of any clinical interest are almost always heavier than that. Third, efflux transporters (particularly P-glycoprotein, encoded by the ABCB1 gene) actively pump molecules back out of enterocytes into the intestinal lumen.

Pharmaceutical chemists have spent decades trying to work around this. Cyclisation of the peptide backbone, N-methylation of amide nitrogens, incorporation of D-amino acids, and PEGylation (attachment of polyethylene glycol chains) are all structural strategies that can meaningfully reduce protease susceptibility and improve membrane permeability. None of them fully solves the problem. They shift the numbers; they don't rewrite the biology.

Why subcutaneous injection changes everything

Bypassing the gut entirely is often the most practical solution. Subcutaneous injection deposits the peptide into the loose connective tissue beneath the dermis, where it enters capillaries and lymphatic channels directly. Absorption from subcutaneous tissue is slower than intravenous delivery but far more complete than oral; bioavailability for subcutaneous peptide administration frequently sits between 70% and near 100%, depending on the molecule's physicochemical properties and formulation.

The rate of absorption from subcutaneous tissue depends on local blood flow, the molecular weight of the peptide, and whether the formulation contains any depot-forming excipients designed to slow release. Intramuscular injection offers faster absorption than subcutaneous, driven by greater vascularity in muscle tissue, but subcutaneous is generally preferred for self-administration because it is technically simpler and has a lower risk of inadvertent intravascular injection.

Nasal and pulmonary routes are active research areas. The nasal mucosa is well vascularised and bypasses first-pass hepatic metabolism, but absorption is inconsistent for larger peptides and mucus clearance mechanisms limit contact time. Pulmonary delivery (inhaled) faces similar issues plus the challenge of landing the peptide in the alveoli rather than depositing it in the upper airways, which requires tight particle-size engineering.

Distribution: where does the peptide go once it's in circulation?

Volume of distribution (Vd) describes how extensively a drug distributes into tissues beyond the plasma. Small lipophilic molecules can have enormous Vd values, they partition freely into fat, muscle, and organs. Peptides, being hydrophilic and relatively large, tend to have low-to-moderate Vd values. Many circulate largely in the plasma and extracellular fluid, with limited penetration into intracellular compartments.

Plasma protein binding matters here too. Albumin is the main carrier protein in blood, and it binds a wide range of molecules with varying affinity. A peptide that is heavily protein-bound has a lower free (pharmacologically active) fraction in plasma. This affects both its biological activity and its rate of renal filtration. The pharmacokinetics of a given peptide can shift meaningfully depending on the patient's albumin level, something relevant in clinical populations where hypoalbuminaemia is common.

Blood-brain barrier penetration is generally poor for peptides, unless they are actively transported across. The CNS is protected by tight junctions between brain endothelial cells that are even more restrictive than the gut epithelium. Certain neuropeptides have evolved endogenous transport mechanisms that carry them across, but therapeutic peptides designed to act peripherally generally don't need to; and often shouldn't, cross into the CNS.

Metabolism: the peptide is its own metabolite

Here's the thing that I genuinely find elegant about peptide metabolism, and honestly I'll take elegance wherever I can find it in a pharmacology textbook. When a peptide is cleaved by a protease, the products are amino acids. Those amino acids enter the body's normal amino acid pool. They are, broadly speaking, non-toxic. Compare this to hepatic cytochrome P450 metabolism of synthetic small molecules, where the metabolites can be more reactive than the parent compound, and you start to see why peptide therapeutics tend to have a relatively benign metabolic profile.

The main metabolic sites for systemically circulating peptides are the kidneys (particularly for smaller peptides below about 30 kDa, which can be filtered at the glomerulus and then cleaved by brush-border peptidases in the tubules), the liver, and circulating blood itself, plasma contains a variety of peptidases including dipeptidyl peptidase-4 (DPP-4), the very enzyme that rapidly inactivates endogenous GLP-1. The class of GLP-1 receptor agonists was specifically engineered to resist DPP-4 cleavage; that's a pharmacokinetic optimisation problem that took a long time and a lot of structural chemistry to solve.

Elimination half-life and why it varies so widely

The elimination half-life of a peptide; the time for plasma concentration to fall by half, ranges from minutes to days, depending on molecular weight, protease resistance, protein binding, and whether the molecule has been modified with half-life-extending technology. Unmodified endogenous peptides like GLP-1 itself have half-lives under two minutes. Chemically modified analogues of the same peptide can have half-lives exceeding a week.

PEGylation is one of the oldest half-life extension strategies: attaching PEG chains increases hydrodynamic radius, reduces renal filtration, and sterically shields peptide bonds from proteases. Fusion to albumin-binding moieties (or to albumin itself) exploits the FcRn receptor recycling pathway that keeps albumin in circulation for weeks rather than hours. These aren't minor tweaks. They are the difference between a molecule that requires multiple daily injections and one that is clinically usable.

From a regulatory standpoint in Australia, any peptide medicine, whether long-acting or not; must be assessed by the Therapeutic Goods Administration (TGA) and, if approved, listed or registered on the Australian Register of Therapeutic Goods (ARTG). Peptides supplied outside this framework are not approved medicines. The TGA has been increasingly active in enforcement around unapproved peptides marketed for performance or anti-ageing purposes; supplying such products in Australia without TGA approval is illegal under the Therapeutic Goods Act 1989. The Special Access Scheme and Authorised Prescriber pathways exist for unapproved substances prescribed in specific clinical contexts, but these are tightly regulated and clinician-led frameworks, not loopholes for general access.

Comparing peptide PK to small-molecule PK: a brief table of contrasts

It's worth being direct about what makes peptide pharmacokinetics structurally different from the small-molecule world, rather than treating the two as variations on a theme. Small molecules are typically lipophilic enough to cross membranes passively; peptides generally aren't. Small molecules are mostly metabolised in the liver via CYP enzymes; peptides are mostly cleaved by ubiquitous proteases distributed across the gut, blood, kidneys, and liver. Small molecules often have oral bioavailability engineered in during development; peptide oral bioavailability remains a hard problem despite decades of effort. And small-molecule metabolites can be genuinely toxic; peptide metabolites are, in most cases, just amino acids.

None of this makes peptides "better" or "worse" pharmacologically, that framing is too blunt. It makes them different, with a specific set of challenges and, in many cases, genuinely useful properties that small molecules can't replicate. The selectivity of a peptide for its receptor target can be extraordinary. I'd argue this selectivity advantage is still underappreciated in some pharmacology curricula, which tend to treat peptide therapeutics as a footnote to the main event of small-molecule drug development. The clinical pipeline data suggests that's a perspective that needs updating.

Sources

, Priya Nandakumar, Pharmacology writer, peptides & PK

Common questions

Why can't most peptides simply be taken as a tablet or capsule?: Peptides are broken down rapidly by digestive enzymes in the stomach and small intestine, and the intestinal wall physically blocks their absorption due to their size and hydrophilicity. Oral bioavailability for unmodified peptides is typically below 2%, which makes oral administration impractical for most molecules without significant structural modification or specialised delivery technology.
What does 'half-life' mean for a peptide, and why does it matter clinically?: Half-life is the time taken for the plasma concentration of a substance to fall by 50%. For peptides, a very short half-life (sometimes under two minutes for unmodified endogenous peptides) means the molecule is cleared too quickly to maintain any sustained biological effect. Chemical modifications — such as PEGylation or albumin binding — can extend half-life from minutes to days or weeks, which has significant implications for how often a medicine needs to be administered.
Are peptides metabolised by the same liver enzymes as conventional drugs?: Not primarily. Conventional small-molecule drugs are largely metabolised by cytochrome P450 (CYP) enzymes in the liver. Peptides, by contrast, are cleaved by proteases found throughout the body — in the gut lumen, the bloodstream, the kidneys, and the liver. The metabolic products are amino acids, which re-enter the body's normal amino acid pool. This is one reason peptide metabolites are generally less toxic than those of synthetic small molecules, though each molecule still requires individual safety evaluation.
What is the TGA's position on unapproved peptides in Australia?: The TGA enforces the Therapeutic Goods Act 1989, which requires that medicines — including peptides — be assessed and listed or registered on the Australian Register of Therapeutic Goods (ARTG) before supply. Supplying unapproved peptides in Australia is illegal. The Special Access Scheme and Authorised Prescriber frameworks allow access to unapproved substances in specific, clinician-supervised contexts, but these are regulated pathways for individual patient use, not general supply channels.
How does subcutaneous injection improve peptide bioavailability compared to oral delivery?: Subcutaneous injection deposits the peptide directly into connective tissue beneath the skin, bypassing the gastrointestinal tract entirely. The peptide is absorbed into local capillaries and lymphatics without facing luminal proteases or the intestinal epithelial barrier. Bioavailability via the subcutaneous route typically ranges from 70% to close to 100% for most peptides, compared to well under 2% for oral delivery of the same unmodified molecule.