On the bottom of page 11 of the Prescribing Information for Zepbound, the “preservative-free” part is somewhat misleading because it is specifically referring to the active ingredient tirzepatide. “Each single-dose pen or single-dose vial contains preservative-free 0.5 mL solution of 2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg of tirzepatide and
the following excipients: sodium chloride (4.1 mg), sodium phosphate dibasic heptahydrate (0.7 mg), and water for injection.” If you notice the part about excipients (substances used to preserve, stabilize, and/or enhance drugs) those are what I was referring to as preservatives.
As a nurse I wouldn't want to add more volume if avoidable but it isn't necessary to add bacteriostatic water as a preservative for Zepbound. When stored in the refrigerator (2-8C/35.6-46.4F) it is good for 24 months from date of manufacture.