My guess is that, the drug wears off when the body of that person who took it is in water.
- Looking back at this chapter here where Nayuta’s boobs quickly shrink in the sea beach, also evidence by Mihari says the drug Nayuta took is a modified version and wouldn’t last long (chap 57 p4, chap 58 p5).
- Chap 17, 18 where Mahiro grew his pp back in the onsen.
- Chap 49, 50 Mahiro grew his pp again in the pool.
(Assuming that Mahiro has contacted water before and definitely has taken the stronger drug.)
But hey that just a theory, A MANGA THEORY!