Doctor Benjamin Carlson is a character in DEXTER. He was a physician assigned to Travis Marshall, on whom he wrote a psyche evaluation. According to Carlson, Travis suffered from violent tendencies since childhood, lack of empathy, was a manipulator and had delusions of grander. Carlson even believed that Travis was responsible for the deaths of both of his parents, having grabbed the steering wheel to force their car into a tree killing both of them during his childhood.
Carlson would prescribe a medication known as Chlorpromazine to Travis. The medication, which is a form of anti-psychotics, however was not efficient, as Travis eventually killed a man named Professor James Gellar and later became the Doomsday Killer.
Carlson passed away in 2009, of unknown causes. It is likely that Travis still received the prescription medicine or eventually he decided to no longer take it.