It's hard not to get an *overwhelming* feeling that Nayuta is in the same situation to whatever degree that Mahiro is. The smoking gun is when Mihari says to her senpai in 22.5 "If you mean Onii-chan, then he safely became a middleschooler thanks to the groundwork you've already set up"
... groundwork. As in, foundational. As in, they already had solid data. It doesn't sound like Mahiro is the first time they've used this drug.
And now we see that Nayuta is well familiar with Mihari's senpai, to the point of already knowing Mahiro's secret in advance.