The Scumbag Professor (1966)

A guy who calls himself a professor claims he's invented a drug that brings youth back to old people. He picks an old man and tests his drug on him. At the same time, a baby is left with him to look after, but the professor, who's lost his focus, thinks that using too much of the drug has turned the old man into a baby. His reaction to this incident eventually lands him in a mental hospital.