Is retatrutide at 20 mg/mL stable enough for sixteen weeks of multi-withdrawal use?

Setup, so nobody has to ask: retatrutide · 20 mg/mL · sixteen weeks.

This is asserted often enough that I assumed it was established, and then I went looking for the source.

I have searched the primary literature and found one paper that is adjacent but not on point.

Can anyone point me at a primary source, or confirm that there is not one?

sixteen weeks is 112 days and, on a weekly schedule, 16 stopper punctures out of one vial at 20 mg/mL. Set the chemical question aside for a moment, because the puncture count is the one with a convention attached: 112 days is 4 times the twenty-eight days conventionally allowed for a preserved multi-dose preparation once it has been entered. Chemically, 20 mg/mL is high enough that adsorption to the glass is a rounding error and low enough that it is not protecting you from anything. What 16 withdrawals do add is 16 opportunities to introduce air, 16 coring events on the same stopper, and a headspace that grows with every draw — none of which show up on a certificate and all of which are avoided by splitting into aliquots at reconstitution.

This is answerable from the chemistry rather than from anecdote, which is unusual and welcome.

Aggregation is physical: peptides unfold at air-liquid interfaces and associate. Shaking maximises that interface, which is why swirling and shaking produce visibly different outcomes on the same vial.

Adsorption onto glass and plastic is significant at low concentrations — micrograms per millilitre — and negligible at milligrams per millilitre. It is the usual explanation for an apparent loss in a dilute preparation.

Metal-catalysed oxidation of methionine is documented across peptide and protein formulations and is why chelators appear in some formulations.

Swirl, never shake. Aggregation is a handling problem more than a time problem.

edited 22 Aug 2026 by triple_agonist_q — updated for the 2026 guidance change

Answering this needs the physical state, since a dry powder is protected from most of these and a solution is protected from none.

Oxidation targets methionine, cysteine and tryptophan, adding sixteen daltons per oxygen. It is catalysed by trace metals and promoted by dissolved oxygen and by light.

To be exact about it, freeze-thaw cycling drives aggregation through concentration at the ice interface and pH shifts as buffer components crystallise out at different rates. Each cycle costs something.

Aggregation at air-liquid interfaces is established from surface-tension and particle-count studies and is the basis for anti-agitation handling guidance.

Nothing here is medical advice, and research-use compounds are not approved for human use.

At dilute concentrations, suspect adsorption before you suspect chemistry.

Start with the sequence, because which pathways are available depends on which residues are present.

Deamidation converts asparagine or glutamine to the corresponding acid via a succinimide intermediate, adding one dalton. It is base-catalysed, accelerates above neutral pH and is the dominant aqueous pathway for many peptides.

A mass spectrum resolves most of this: minus eighteen is dehydration or succinimide, plus one is deamidation, plus sixteen is oxidation, and an unchanged mass with a shifted retention time is an isomer.

Adsorption losses at low concentrations are quantified in formulation studies and are the reason carrier proteins are used in dilute preparations.

A mass spectrum names the pathway. Plus one, plus sixteen, minus eighteen.