Possibly Research Alpha by Van Vogt?
A synopsis I found:-
A Research Alpha scientist decides to test a serum he has developed to expedite the evolution of man’s mental abilities. He first injects it into a typist who works for the laboratory’s chief administrator, then into her boyfriend. He hopes to watch them evolve together and perhaps create offspring.
The young woman immediately notices she is smarter and more mentally sharp. She suspects that the strange pinch she felt on her shoulder when the doctor was standing behind her has something to do with what is happening to her. Her boyfriend, whose shot didn’t quite take, becomes lethargic and ill.
The scientist keeps his subjects under close observation and decides to inject them with a second, higher level serum. The young woman’s mental powers increase dramatically and draw the attention of the head of Research Alpha and his assistant who try to monitor the woman’s evolution to determine if she will act as an agent of good or an agent of evil.
The boyfriend also receives a second injection, but his condition deteriorates. The scientist decides he must die and tries to kill him by drowning him. But the leaders are able to save him and bring him back to the laboratory.
Meanwhile, the young typist finds her new mental abilities astounding. She is able to send her consciousness out into the universe to observe cosmic events. She can delude people into believing whatever it is she would have them believe. While she is not evil, she is undisciplined and the heads of Research Alpha – they themselves mentally superior beings – decide they must reign her in. They set a trap for her, using the boyfriend as bait, to bring her back to the lab.
She returns to the lab, knowing that the snare is set, but not caring. She has enough humanity in her to be concerned about her boyfriend. The lab director shows her her boyfriend who has physically devolved into a 22 inch tall, dwarvish creature. The lab director explains that the serum effects the genetic code of every person differently because every person has a different genetic code.