The 28-day guideline is a conservative benchmark built on pharmaceutical stability data that applies to compounded preparations without preservatives. Reconstituted peptides kept in the fridge with proper sterile technique - alcohol pads, single-use syringes, minimal needle penetrations - regularly hold well beyond 28 days in community experience. The main degradation factors are temperature fluctuations (avoid the fridge door), repeated freeze-thaw cycles, and contamination from multiple draws. Seven weeks on a Reta vial with consistent cold storage and clean technique is within what experienced users report without potency issues. The 28-day number is where manufacturers stop testing, not where peptide activity stops.