One of the most common complaints of people who take anti-depressants is that they tend to feel emotionally numb or experience “emotional blunting” and no longer find their hobbies or other activities as pleasurable as they used to. In a new study, a group of scientists found that a selective serotonin reuptake inhibitors (SSRI) can affect reinforcement learning, which refers to a person’s decision-making abilities. Reinforcement learning is a set of learned behaviors based on interactions with the environment and observations that can help an individual to achieve goals or rewards. Researchers estimate that between 40% to 60% of people who take antidepressants experience this side effect.
In England itself, a 2022 study revealed that the number of antidepressants that doctors prescribed to their patients increased by almost 35% in the last six years. According to the Centers for Disease Control and Prevention, from 2015-2018, at least 13% of adults in the U.S. overused antidepressants. That was more common among women (17.7%) than men (8.4%).
To better understand the impacts of long-term use of SSRIs, researchers from the University of Cambridge and the University of Copenhagen recruited 66 healthy participants. They prescribed a common antidepressant called escitalopram to 32 of them and the other 34 were given a placebo. Each participant took these pills for at least 21 days and was also made to complete a set of questionnaires that assessed their personality traits, depressive symptoms, anxiety, and impulsivity. Then, the participants completed two tests evaluating their cognitive functions. This included their decision-making skills, learning abilities, and how they were planning, prioritizing, and executing their daily routine tasks.
While the team did not find any differences in their memory and attention, the group that was taking 20mg of escitalopram regularly had lower reinforcement sensitivity compared to the placebo group. The results, which were published in the journal Neuropsychopharmacology, were based on the participants’ ability to choose stimuli that would give them a reward.
Between stimuli A and B, if the participants chose A, four out of five times that would result in a reward. Whereas choosing B would result in a reward only one out of five times. The participants had to figure this out by themselves. The researchers also kept switching which stimuli would yield more results throughout their experiment to see whether all 66 participants could learn that new rule or not.
While participants on placebo learned the tasks through the positive and negative feedback they received, those who were taking the antidepressant were less likely to have the same level of sensitivity to rewards. And that impacted their ability to respond to the stimuli.
“Participants receiving seven days of an SSRI had lower neural processing of both rewarding and aversive stimuli,” the researchers observed.
“In light of our own results, it is possible that the clinical effectiveness of SSRIs for major depressive disorder is due to this reduced negative effect. However, if indeed positive affect is also reduced, then this would lead to a more general blunting effect, as often reported by patients taking chronic SSRIs,” the researchers explained in their study. “This is supported by the present study, in which lower reinforcement sensitivity would suggest decreased control over behavior by both rewarding and punishing stimuli.”
In simpler words, that means while SSRIs take away some of the emotional pain that depressed people experience, they also take away some of the enjoyment, said Barbara Sahakian, senior author and a professor from the Department of Psychiatry at the University of Cambridge, in a press release.
The researchers further hypothesized that participants taking escitalopram might also experience more sexual dysfunction because they experience less pleasure. While this has been supported by previous studies, the researchers added: “This is speculative as there are other mechanisms that may explain this effect.”