Why antidepressants can blunt both negative and positive emotions

An unwanted flattening of all emotions is one of the most frequent side effects of antidepressants – and now we know more about why it can happen.

The most commonly used types of antidepressant belong to a class called selective serotonin reuptake inhibitors (SSRIs). These are thought to work by increasing levels of the brain chemical serotonin, although why this can improve our mood is unclear.

Up to half of people taking antidepressants experience an unwanted dampening of both positive and negative emotions. “They talk about not feeling much,” says Barbara Sahakian at the University of Cambridge.

Depression itself also often causes a lack of pleasure in activities a person once enjoyed. Sahakian and her colleagues therefore investigated an SSRI’s emotion-dampening effect in people without the mental health condition.

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The researchers gave either a commonly prescribed SSRI called escitalopram or placebo pills to 66 people without depression. After three weeks, the participants carried out a range of tasks involving memory and learning.

One task measured how well they learned from rewards, with people having to repeatedly choose between two stimuli. Through trial and error, they generally learned that one stimulus led to a reward more often than the other. Then the probabilities of a reward for each stimulus would switch and the participants had to learn this new system.

The participants who took the antidepressant were 23 per cent less sensitive to the stimuli switch than those taking the placebo, as measured by how quickly they changed their stimulus selections. Other tests showed that the medicine didn’t reduce their cognitive abilities in other ways.

The finding suggests that SSRIs reduce people’s sensitivity to rewards or other pleasurable experiences, says Sahakian. But the medicines can also blunt the intensity of negative feelings, which can be helpful, she adds.

“I hope this doesn’t make doctors more cautious about prescribing antidepressants as they’re extremely important drugs,” she says. “I hope it would make doctors have a discussion with patients about potential side effects.”

“Why antidepressants cause emotional blunting in a subset of people is a really important question,” says Catherine Harmer at the University of Oxford. “I don’t think this result explains why people have this effect, but it may be a marker of it, which could be useful when we come to develop new treatments that don’t have it.”

Harmer says the study would have been more useful if the participants had also been asked if they experienced blunted emotions while taking the antidepressant.