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Looking at the pictures [1] I can kind of tell that it was the right side of my mouth (left side of the pictures) that I was putting chapstick on. While the left side of the mouth consistently looks slightly drier, it doesn't get worse and worse. So no medium term effect and only minimal short term effect. I'm going to think of chapstick more as a temporary relief thing from now on, and if it doesn't make my lips feel better not use it.

[1] Each of these pictures was taken in the same conditions: the bathroom at work, in the same lighting, with the camera on my phone. I applied chapstick as I normally would, which generally meant "when my lips felt dry". I do like that the timing worked out such that I'm posting huge numbers of pictures of lips on Valentines day.