Why antidepressants can blunt both negative and positive emotions

Escitalopram, a commonly prescribed antidepressant, reduced people’s sensitivity to rewarding experiences in a small trial

To Claire Wilson

An unwanted flattening of all emotions is one of the most common side effects of antidepressants.

The most commonly used type of antidepressant belongs to a class called selective serotonin reuptake inhibitors (SSRIs). These are thought to work by increasing levels of the brain chemical serotonin, but it’s unclear why this can improve our mood.

Up to half of people taking antidepressants experience an unwanted attenuation of both positive and negative emotions. “They say they don’t feel much,” says Barbara Sahakian of the University of Cambridge.

Depression itself is often the cause of a lack of pleasure in activities that were once enjoyed. Sahakian and her colleagues therefore investigated the affect-suppressive effects of SSRIs in people without mental health conditions.

The researchers gave 66 people without depression either a commonly prescribed SSRI called escitalopram or a placebo tablet. After three weeks, participants performed a variety of tasks related to memory and learning.

One task required repeatedly choosing between two stimuli and measured how well they learned from the reward. Through trial and error, they learned that some stimuli usually yielded rewards more often than others. The reward probabilities for each stimulus were then switched, and participants had to learn this new system.

Participants taking antidepressants were 23% less sensitive to stimulus switching than participants taking placebo, as measured by how quickly they changed stimulus choice. Other tests have shown that the drug does not impair cognitive performance in other ways.

The findings suggest that SSRIs reduce people’s sensitivity to rewards and other pleasurable experiences, Sahakian said. But the drug can also blunt the intensity of negative emotions, which may help, she added.

“I hope this doesn’t make doctors more cautious about prescribing antidepressants because they are very important drugs,” she says. “We hope it will inspire doctors to discuss potential side effects with their patients,” she said.

“Why antidepressants cause emotional blunting in some people is a very important question,” says Catherine Hamer of the University of Oxford. I don’t think it explains why, but it could be that marker and could be useful in developing new treatments that don’t have it.”

Hamer said the study would have been more helpful if participants were also asked if they experienced dulled emotions while taking antidepressants.