I prefer not to keep more than four weeks of reconstituted material in one container. In this case I'd split the twelve-week supply into three separate four-week portions, freeze two, and thaw them when needed. Freezing these peptides hasn't caused noticeable loss of effect for me. I haven't tried freezing pen cartridges themselves, though I'd test it first with just bac water.
I've moved from pharmacy compounded glp-1s into grey-market peptides and now use pens for the latter. Still working through my remaining pharmacy tirzepatide that arrived in 3 ml and 5 ml liquid vials. Transferring that liquid into pen vials should be fine since the vials are already multi-dose, but the fact that lower dosing stretches them to about twelve weeks instead of four makes me hesitate over the pens' sterility. Thoughts on whether this is overcautious, whether syringes are safer, or if there's a middle option?