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CAMBRIDGE, United Kingdom — Common antidepressants can cause users to experience “emotional dullness,” blunting their ability to gain pleasure from everyday experiences, a new study warns. The widely-used class of antidepressants, which doctors commonly prescribe for persistent or severe cases, belong to the drug group selective serotonin reuptake inhibitors (SSRIs). These drugs target serotonin, a chemical that carries messages between nerve cells in the brain, which scientists call the “pleasure chemical.”

According to the Centers for Disease Control and Prevention, more than 13 percent of U.S. adults took antidepressants between 2015 and 2018. However, one of the widely reported side-effects of SSRIs is “blunting,” where patients feel emotionally dull and stop finding things as pleasurable as they used to.

Between 40 and 60 percent of patients taking SSRIs may experience this side-effect, according to estimates. Antidepressants can also have an effect on sex. People who take escitalopram, one of the best tolerated SSRIs, often report that they struggle reaching orgasm when having sex.

For the study, a team from the University of Cambridge recruited 66 healthy volunteers. Out of that group, 32 received escitalopram, while the other 34 took a placebo. They took the drug or fake pill for at least 21 days. During this time, participants completed a comprehensive set of questionnaires and took a series of tests assessing their cognitive functions. These included learning, inhibition, executive function, reinforcement behavior, and decision making.

Antidepressants may also prevent patients from learning from past events

The team found no significant difference when it came to attention and memory. There were also no differences in most tests that assessed cognitive functions that involved emotions. However, taking the drug did reduce participants’ sensitivity to rewards and their ability to respond to them during two of the tasks.

Those taking escitalopram found it harder to learn from feedback from their actions and environment. They were less likely to use positive and negative feedback to guide their learning. These results were clear during a reversal test as well. In this task, a participant was shown two stimuli, named A and B. If they chose A, then four out of five times they would get a reward. If they chose B, they would only receive a reward one time out of five.

The volunteers were not told this rule, but they would have to learn it themselves. At some point in the experiment, the probabilities would switch, meaning the participants would need to learn the new rule. Those taking the drug found it much harder to use the positive and negative feedback to guide their learning.

“Emotional blunting is a common side effect of SSRI antidepressants. In a way, this may be in part how they work – they take away some of the emotional pain that people who experience depression feel, but, unfortunately, it seems that they also take away some of the enjoyment. From our study, we can now see that this is because they become less sensitive to rewards, which provide important feedback,” says senior author Professor Barbara Sahakian, of Cambridge’s Department of Psychiatry and a Fellow at Clare Hall, in a university release.

“Our findings provide important evidence for the role of serotonin in reinforcement learning. We are following this work up with a study examining neuroimaging data to understand how escitalopram affects the brain during reward learning,” concludes Dr. Christelle Langley, also from the Department of Psychiatry.

The results of the study are published in the journal Neuropsychopharmacology.

South West News Service writer Alice Clifford contributed to this report.

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