Bioavailability: What It Means and Why Route Matters
What bioavailability means, why peptides taken by mouth reach the blood in tiny amounts, and how label figures differ by product and route for semaglutide, desmopressin and teriparatide.
Content last updated . This page has not been medically reviewed.
What the word means
Bioavailability is the share of a substance that reaches the bloodstream in its active form. Medicine labels report "absolute bioavailability," which compares a route such as an injection under the skin with an injection into a vein. A number belongs to one product given one way. Peptides are a good case study: the label figures below run from well under 1% to about 95%, depending on product and route.Figures from medicine labels
| Product and route | Bioavailability in the label |
|---|---|
| Semaglutide tablets with an absorption helper (SNAC) | About 0.4% to 1% for the Rybelsus tablets, and 1% to 2% for the Ozempic tablets [1] |
| Desmopressin tablets | About 0.16% compared with an intravenous injection, and about 5% compared with the nasal form [2] |
| Liraglutide, injected under the skin | About 55% [3] |
| Tirzepatide, injected under the skin | About 80% [4] |
| Teriparatide, injected under the skin | About 95% [5] |
| Octreotide, injected under the skin | Absorbed rapidly and completely; the label reports it as bioequivalent to the intravenous route [6] |
Two lessons follow. First, the same family of molecules spans a huge range. Second, tablets of peptides exist, but only because of special formulation work and, in the semaglutide tablet label, a helper ingredient (SNAC) that "facilitates the absorption" mainly in the stomach. [1]
Why swallowed peptides mostly fail
A review of oral delivery says the harsh gut environment and poorly penetrating gut lining often limit oral bioavailability of therapeutic peptides and proteins to less than 1% to 2%. [7] A food-science review reaches the same type of conclusion for bioactive peptides from food, citing taste, stability in the gut and transport across the gut wall as barriers. [8] See why peptides are often broken down during digestion and oral peptide medicines.How the absorption helper works is still debated. One comparison of SNAC and another enhancer found that tablets with these helpers gave single-digit, highly variable increases in oral bioavailability in human trials, which "may be adequate for potent macromolecules." [9] In plain words: a very small share can be enough only when the molecule is potent and the product was developed and tested for that purpose.
Route matters, and so do the details
- Under the skin or into a vein. Bypasses the gut, so more reaches the blood. Even then labels show 55% to 95%. [3][5]
- Through the nose. Desmopressin has an intranasal form in its label comparison. [2] A review of nose-to-brain delivery lists poor permeation through the nasal lining and mucociliary clearance as limits, and says few such products have won approval for lack of sufficient clinical evidence. [10] Reaching the brain from the nose is not something to assume; see intranasal peptide research.
- Details of use. The semaglutide tablet label reports that absorption in studies differed with the volume of water and the length of fasting afterward. [1] It illustrates how sensitive a low-bioavailability product is to conditions. It is not guidance for anyone.
Bioavailability is not the same as benefit
A high number does not show that a product helps, and a low one does not show that it cannot. Benefit depends on how much reaches the target and what happens next. For a closer look see pharmacokinetics vs clinical effectiveness. Also see encapsulation, liposomes and nanoparticles, because the word "liposomal" on a label is not evidence of absorption.Reading claims safely
- A bioavailability number should name the exact molecule, formulation, route and population.
- A figure from a prescription label for one product cannot be applied to another product, even one with a similar name; see why similar names do not mean equivalent products.
- "Absorbs better," "sublingual" or "nano" on a product page are marketing claims unless backed by human data on that product.
This page is education only.
Frequently asked questions
Why can some peptides be swallowed and others cannot?
Most are broken down or not absorbed in the gut. A few tablets work because of formulation features. The semaglutide tablet label reports bioavailability of roughly 0.4% to 2% with an absorption helper. [1]
What is a "good" bioavailability?
There is no single target. Teriparatide injected under the skin has about 95%, while desmopressin tablets have about 0.16% compared with intravenous use and are still used as medicines. [5][2]
Does a nasal spray carry peptides to the brain?
Not reliably. A review notes limits such as poor passage through the nasal lining, and few approved products. [10]
Does a lower bioavailability make a product unsafe?
Not by itself. It makes results more variable, which is a reason for product-specific testing. [9]
References
- U.S. FDA prescribing information, oral semaglutide tablets (RYBELSUS and OZEMPIC tablets), sections 12.1 and 12.3, label effective 2026-01-30, read as the FDA label record at openFDA drug label API. Read 2026-10-22.
- U.S. FDA prescribing information, DDAVP (desmopressin acetate) Tablets, description and clinical pharmacology, label effective 2021-02-03, read as the FDA label record at openFDA drug label API. Read 2026-10-22.
- U.S. FDA prescribing information, Victoza (liraglutide), sections 12.1 and 12.3, label effective 2023-02-06, read as the FDA label record at openFDA drug label API. Read 2026-10-22.
- U.S. FDA prescribing information, Mounjaro (tirzepatide), sections 12.1 and 12.3, label effective 2026-07-29, read as the FDA label record at openFDA drug label API. Read 2026-10-22.
- U.S. FDA prescribing information, Forteo (teriparatide), sections 12.1 and 12.3, label effective 2026-08-03, read as the FDA label record at openFDA drug label API. Read 2026-10-22.
- U.S. FDA prescribing information, Sandostatin (octreotide) Injection, sections 11 and 12, label effective 2026-07-22, read as the FDA label record at openFDA drug label API. Read 2026-10-22.
- Han Y, et al. Multifunctional oral delivery systems for enhanced bioavailability of therapeutic peptides/proteins. Acta Pharm Sin B. 2019. PubMed 31649842 (abstract). Read 2026-10-22.
- Wang X, et al. Obstacles, research progress, and prospects of oral delivery of bioactive peptides: a comprehensive review. Front Nutr. 2024. PubMed 39610876 (abstract). Read 2026-10-22.
- Intestinal Permeation Enhancers for Oral Delivery of Macromolecules: A Comparison between Salcaprozate Sodium (SNAC) and Sodium Caprate (C10). Pharmaceutics. 2019. PubMed 30781867 (abstract). Read 2026-10-22.
- Nose-to-brain drug delivery: An update on clinical challenges and progress towards approval of anti-Alzheimer drugs. J Control Release. 2018. PubMed 29772289 (abstract). Read 2026-10-22.