If your bra is uncomfortable, then you might not have been correctly fitted (or may no longer fit your bras). Probably half of all women (who wear bras) wear the wrong size. The most common error I see is a band zig-zagging across a woman's back. That means a band size too big, and the shoulder straps have been tightened to compensate (ouch!).
A misfit bra isn't doing its job, and it's transferring most or all of its stress to your shoulders, so get rid of it. However, don't give up on the technology completely unless you're sure you've tried a bra that's really your size. The band should run straight and snug (but not tight) around your torso, and you should not overflow the cups. If brick-and-mortar stores can't do that for you, then search online. While you're there, read up on how to correctly measure yourself. You may never trust a store clerk again.
asiento-trasero
there have been studies done linking bras and breast cancer.
At least part of that could be ill-fitting under-wire, which can clamp the lymph duct that's supposed to carry lymph from nodes in the breast to a node in the arm-pit (on its way to the spleen for disposal). Lymph carries metabolic trash, free-radicals and some carcinogens. Clamping the duct traps that crap in the nodes within the breasts, so increased cancer risk would not be surprising.
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