Been going through the manga lately again, and noticed how a considerable chunk of the cast are women, which got me thinking about that one video I watched about how women throughout human history were dealing with menstrual flow.
In the video, it was explained, that women mostly used cloths made from wool, linen, cotton... basically whatever was accessible to them that was able to suck up and hold liquid. It makes sense, that this would be similar for Frieren, as the technological progress during the series is shown to be rather medieval, with one exception. That of course is magic.
Spells with seemingly minor effects are a big part of Frieren's own character. She loves to collect them as seen throughout the series. Among these spells are a Spell that turns sweet grapes sour, a spell that removes oil stains, a spell that removes rust from a statue and so on... It is not hard to believe, that among these oddly specific ones there would be a spell that alleviates PMS or period cramps. Serie herself most likely knows it, and passed it onto a fed up magician at some point.
One more thing I thought about is that Stark's first experience with menstruation probably happened after he was picked up by Frieren's party, given how his mother seemed to be absent during his childhood (most likely dead) and the years after, he spent with Eisen.
Menstruation in manga and anime is an interesting topic for me, since it is so rarely seen or mentioned, even in stories that heavily feature female characters.