Super-Special Tummy Tonic
From the Super Mario Wiki
At the beginning of her story in the Twist League, Penny creates an elixir with the desire of using it to enhance her voice and become a pop star. She then tests it on her grandfather, Dr. Crygor, and it is successful in making his voice higher in pitch, but also puts him in a condition of extreme stomach pain.
Penny, remembering her Super-Special Tummy Tonic, gives it to her grandfather to use as a cure for the Tonally Transformative Elixir. Although it reduces Dr. Crygor's stomach pains, it does not fix his higher-pitched voice, which is suggested to be a permanent effect.
Regardless, Dr. Crygor speaks with his regular voice in future scenes, suggesting that the effects might have gone away on their own or been cured by another potion.