Insulin is the archetype. It is the peptide hormone - 51 amino acids across two chains, A and B, held together by disulphide bonds - that made peptide therapeutics possible, and it remains the peptide class in which every member in UK clinical use is a licensed prescription medicine rather than a research compound.

That fact is the story here. For most compounds catalogued on this site the question is what the evidence shows and why there is no UK licence. For insulin the question is which of three generations of product a given preparation belongs to, and what structural change produced its time-action profile.

What the molecule is

Human insulin is a 51-residue peptide hormone: an A chain of 21 amino acids and a B chain of 30, cross-linked by two inter-chain disulphide bonds and one intra-chain bond. It is produced in the pancreatic beta cell as preproinsulin, cleaved to proinsulin and then to insulin plus C-peptide. C-peptide is released in equimolar amounts and is itself catalogued separately on this site.

Recombinant human insulin

Animal-pancreas insulin was replaced in the 1980s by recombinant human insulin expressed in E. coli or yeast - a molecule identical in sequence to the human hormone. It remains in UK use, and it is the reference against which the analogues are compared: it self-associates into hexamers in solution, and the time those hexamers take to dissociate after administration is what limits how quickly it can act.

The rapid-acting analogues

Three analogues - insulin aspart, insulin lispro and insulin glulisine - modify the B chain to reduce self-association. Lispro swaps the B28 and B29 residues; aspart replaces proline at B28 with aspartic acid; glulisine alters B3 and B29. The consequence is a molecule that stays monomeric more readily, is absorbed faster and has a shorter duration of action than human insulin. The differences between the three are small; the class difference from human insulin is the point.

The long-acting analogues

The long-acting analogues achieve the opposite through three distinct mechanisms. Insulin glargine has an altered isoelectric point - raised by an A21 substitution and two added B-chain arginines - so that an acidic formulation precipitates at physiological pH in subcutaneous tissue and is absorbed slowly. Insulin detemir carries a myristic-acid side chain at B29 and binds serum albumin, extending its circulation time. Insulin degludec forms multi-hexamers that assemble into a soluble depot. These are three different pieces of medicinal chemistry arriving at the same idea: a flatter, longer profile.

Combination products also exist in UK use, pairing a long-acting insulin with a GLP-1 receptor agonist - for example insulin degludec with liraglutide - which places insulin alongside the incretin peptides in the same glucose-lowering space.

The UK licensing picture

Every insulin and insulin analogue in UK use holds a marketing authorisation and is a prescription-only medicine under the Human Medicines Regulations 2012, which implement the Medicines Act 1968 framework and are enforced by the MHRA. None is a 'research use only' compound, and none is sold legitimately as a research chemical. This is the mirror image of the grey-market norm across much of the rest of the peptide field, and it is why insulin is worth documenting as a class: it shows what a fully regulated peptide medicine looks like.

Evidence grade

Strong. The class is among the most extensively studied in medicine, and insulin appears on the WHO Model List of Essential Medicines. The comparative questions that remain - nocturnal hypoglycaemia, weight effects and long-term outcomes between individual products - are refinements within a well-established evidence base rather than disputes about whether the class has one.

What this article is not

This is a pharmacology and regulatory explainer. It gives no dosing, no administration or injection guidance and no personal-use advice, and it is not a treatment recommendation. Insulin is a prescription medicine whose use is individual and managed by a clinician. Nothing here describes or endorses the use of research peptides, which are a separate category under UK law and are not medicines.

This article is AI-researched and editorially reviewed. It is provided for research and educational purposes only and is not medical advice. Research peptides are not licensed for human consumption in the UK.