Prescribed vials are safe for more than a month, and the preservatives in them last longer than a month (this is also true for DIY vials, which are often used for as long as a year or more). They can be compromised any time, though - so it makes sense still to refill your Rx once a month. (My endo originally didn’t have me refill my vials once a month and expected me to use my vials for many months, he had no concerns about the safety, but was willing to write the Rx to refill once a month based on my argument that the FDA recommends pitching vials after a month. This was especially a concern I had after I cored some vials and was going to run out of medication.)

I use prescribed vials (100 mg estradiol valerate in 5 mL) for 3+ months without issue, they are more likely to core and be compromised that way than to become unsafe from pathogens from drawing (because of the preservatives). This makes it more important that you use thin needles for drawing, and use proper technique to avoid coring. Of course this is also why it’s so important to wash your hands before starting the injection, why you sanitize the vial with an alcohol swab, and why you don’t re-use needles, etc. Sterile needles going into a sterilized rubber stopper won’t be a big risk, and the air that has pathogens will be made safe by the preservatives.

The current vial I’m using in October was opened in February (8 months so far).