Pharmacists are taught that 28 days is the maximum after a vial has been punctured. Theoretically the first time you let air in the vial is no longer “sterile”, but I use them until they’re empty and have never had an issue. My endo who specializes in HRT recommends the same and has never had an issue using within 6 months.

Theoretically 28 days thing is really from manufacturers not testing past that threshold and wanting to sell more vials.

Now if you don’t crack/puncture it could be a 10 year old vial and be “safe”. Drugs degrade at rates way slower than most think.

From CDC.gov//United States Pharmacopeia General Chapter 797

If left out of sun light (direct) I’ve seen it last 10+ years depending on the brewer, if cracked I would use asap 6 months at the longest might be able to go longer but I’m not exactly sure and don’t want to guess.

It would depend on the compound. Some have longer than others.