The discovery underneath this whole research area is a good one. Glucose taken by mouth produces far more insulin than the same glucose given intravenously, which means the gut signals ahead to the pancreas before the sugar arrives. That is the incretin effect, and the compounds here are built to engage it.
The receptors involved
Two incretin hormones do most of the work: GLP-1 and GIP. Both are released by the gut in response to food, both act on the pancreas, and they are not interchangeable.
The engineering question that produced this class of compound was whether a single synthetic peptide could engage those receptors in a controlled way and stay in circulation long enough to be studied properly.
Dual and triple agonism
Tirzepatide binds two receptors. Retatrutide binds three, adding the glucagon receptor, which brings energy expenditure into the picture alongside incretin signalling.
Whether three targets prove better than two is an open research question rather than a settled one.
L-carnitine
Sits in this area for a different reason. It is not an incretin compound at all, but a molecule involved in moving fatty acids into mitochondria for oxidation, which places it in metabolic research by function rather than by mechanism.
Regulatory position in the UK
Several compounds in this area are licensed prescription-only medicines in the UK. We state that clearly on each product page. All are supplied as laboratory reagents for in vitro research.