So I'm im no way an expert and am very open to be corrected. I also have only used tirz and not reta, but as I understand it they both are similar in storage hardiness.
I think that the 28 day rule came from the fact that you should throw away a bottle of hospira 28 days after you first puncture it if you go by the instructions for the bac water.
I personally have used reconned bottles into three months. That's not advised but I have. Eventually I got a red itchy spot at the injection site and that was my cue to toss both the bottle of bac and the bottle of tirz. I often use hospira brand bac water that's 3 months from first puncture before I toss it. I then use the tirz bottle for a month before it's used up. That's four months on the bac water since first puncture.
I think the idea is to not let the bacteria start to build up in the bottle. The med will also lose some of its effects after a while but that's not a danger to you.
My advice is to purchase the vials in a size that you will use up in 4-6 weeks. If you go longer it's no big deal. If you have an injection site reaction then it's time for new bottles.
Also how long your bottle will last depends on your hygiene with reconstituting the bottle and pulling doses from it. Be sure to clean with alcohol.
