Because of risk of contamination. Theres 3 things you gotta look for:
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crystallization, means it got too cold and the estradiol is crystalizing out of solution. Give it a shake, usually fixes this issue and ONLY THIS ISSUE
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noticeable flakes/flecks in solution, this is part of the rubber stopper, and you SHOULD NOT USE IF THIS IS A THING
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discoloration or seperation, this means it got too hot and either discolors or the solution breaks entirely (think oil and water, and how one floats above the other, no matter how much you mix it)
Past this, the 6 months rule is for two reasons. One, for contamination reasons, the more you stab at it and do stuff, the more chance of you introducing bacteria even if your careful, which is REALLY bad. Two the efficacy of the drug diminishes, lile any other drug, over time. So theres a chance if you use a 6 month vial that the estradiol isnt as potent.
These dont happen every time you touch your vial, but its enough of a risk that i highly recommend staying to the 6 months thing, and i wont tell people to go past that, for my sake.