Commonly prescribed antidepressants can make patients less sensitive to rewards – affecting a key behavioral learning process that can lead to emotional lethargy, according to scientists.
Researchers have found that selective serotonin reuptake inhibitors, or SSRIs, can affect reinforcement learning, which allows people to learn from their actions and their environment.
These drugs work by targeting the body’s “feel-good” chemical known as serotonin, which carries messages between nerve cells in the brain.
A widely reported SSRI side effect is “blunting”, where patients say they feel emotionally numb and are not able to respond with the same level of pleasure that they normally do.
The experts said their findings, published in the journal Neuropsychopharmacology, shed light on how serotonin affects reinforcement learning.
Professor Barbara Sahakian from the Department of Psychiatry at the University of Cambridge – who is a senior author on the study, said: “Emotional blunting is a common side effect of SSRI antidepressants.
“In a way, it may be part of the way they work – they take away some of the emotional pain that people with depression experience, but unfortunately, it seems they also take some of the pleasure out of it.
“From our study, we can now see that this is because they become less sensitive to rewards, which provide important feedback.”
The researchers recruited 66 volunteers to participate in the experiment, 32 of whom were given escitalopram while the rest were given a placebo.
All participants completed a comprehensive set of self-report questionnaires after 21 days and were tested on cognitive functions including learning, inhibition, executive function, reinforcement behavior and decision making.
The results indicated that reinforcement sensitivity was lower on the two tasks for the escitalopram group compared to placebo.
The researchers said participants taking escitalopram were less likely to use positive and negative feedback to guide their learning of the task than those taking the placebo.
This suggests that the drug affected their sensitivity to rewards and their ability to respond accordingly, the team said.
But other experts caution that patients who are taking SSRI drugs should not stop taking them based on this research.
Commenting on behalf of the Royal College of Psychiatrists, Professor Carmine Pariante, who was not involved in the study, said: “This is an interesting and well-conducted study in healthy subjects, but it does not change our understanding of antidepressants.
“People who are depressed may struggle to feel positive emotions such as happiness which makes it difficult to differentiate between the effects of the condition and the effects of the drug.
“By reducing negative feelings, antidepressants can help people feel better.”
She said antidepressants are an effective form of treatment for people experiencing depression that has a detrimental effect on their quality of life and where other treatments, such as talking therapies, have not worked.
Professor Pariante said: “Clinicians should always discuss with their patients the potential risks and benefits of taking antidepressants because we know that their effectiveness can vary from person to person.
“Physicians should regularly review their use to ensure that they are still needed.
“We would not recommend anyone stop taking their antidepressant based on this study and would encourage anyone with concerns about their medication to contact their GP.”
NHS figures published in July showed 8.3 million patients in England are set to receive antidepressants in 2021/22, up 6% from 7.9 million the previous year.
In 2019, research that looked at nearly 1,000 existing studies published in JAMA Psychiatry concluded that antidepressants are generally safe.
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