It is not exactly bad for your breast health, but it could be bad for your hygiene. Whether you realize it or not, you do sweat around your breast area. Having a tight fitting article of clothing around this area underneath a regular shirt, increases the warmth there obviously too. I wouldn't recommend that you wear the same bra too many days in a row just for good hygiene. After a couple-few days, depending on your body, you should wash it to eliminate any sweat odor. In the winter-time you can get away with wearing the same bra longer than in the summer time. Health wise though, there isn't really any harm.
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