When Nathaniel Schumann first entered the clinical trial for children with autism, he was 8 years old and considered nonverbal. He communicated through gestures and sounds, which his parents were skilled at interpreting. But two weeks after receiving his first dose of the study pill, Nathaniel began to speak — not just words, but full sentences. “The first thing we noticed is kind of funny: He had a laundry list of things that bothered him,” his mother, Kathleen Schnier, said. Anecdotes...