The 28-day lifespan of a multi-use vial is based on guidelines from the CDC regarding contamination and sterility. The retail pharmacies are corporate entities and are going to create corporate policy around those requirements in an effort to do anything to reduce their liability and exposure, so they are going to follow those guidelines.
Technically yes, you introduce contaminants into the vial with every puncture, but that is what the preservative is for. The vial, even a 1mL does not immediately “spoil” at 28 days.
The guidelines are actually intended for a hospital setting.
[URL unfurl="true"]https://www.cdc.gov/injectionsafety/providers/provider_faqs_multivials.html[/URL]
Read carefully and you will notice phrases such as “whenever possible” and “should”, not “must”. Even these guidelines are not absolute as a corporate pharmacist would have you believe. And note #5. It’s a suggested (“should”) 28 days UNLESS a date is otherwise specified by manufacturer on the vial, which they all have.
