Common antidepressants can cause many users to feel emotionally 'blunted', according to a study that offers new insights into how the drugs work and their possible side effects.

The research, published on January 23 in the journal Neuropsychopharmacology, shows that drugs affect reinforcement learning, an important behavioural process that allows us to learn from our environment.

A widely-used class of antidepressants, particularly for persistent or severe cases, is selective serotonin reuptake inhibitors (SSRIs). These drugs target serotonin, a chemical that carries messages between nerve cells in the brain and has been dubbed the 'pleasure chemical'.

One of the widely-reported side effects of SSRIs is ‘blunting’, where patients report feeling emotionally dull and no longer finding things as pleasurable as they used to, the researchers said. Between 40-60 per cent of patients taking SSRIs are believed to experience this side effect, they said.

"Emotional blunting is a common side effect of SSRI antidepressants," said Professor Barbara Sahakian, senior author of the study from the University of Cambridge in the UK.

"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," Sahakian said.

The study shows that this is because people become less sensitive to rewards, which provide important feedback. Most studies of SSRIs to date have only examined their short-term use, but, for clinical use in depression, these drugs are taken chronically, over a longer period of time.

Researchers at Cambridge in collaboration with the University of Copenhagen, Denmark recruited healthy volunteers who were administered escitalopram, an SSRI known to be one of the best tolerated, over several weeks.

They assessed the impact the drug had on their performance on a suite of cognitive tests. In total, 66 volunteers took part in the experiment, 32 of whom were given escitalopram while the other 34 were given a placebo.

Volunteers took the drug or placebo for at least 21 days and completed a comprehensive set of self-report questionnaires and were given a series of tests to assess cognitive functions including learning, inhibition, executive function, reinforcement behaviour, and decision-making.

The team found no significant group differences when it came to ‘cold’ cognition – such as attention and memory.

There were no differences in most tests of ‘hot’ cognition – cognitive functions that involve our emotions. However, the key finding was that there was reduced reinforcement sensitivity on two tasks for the escitalopram group compared to those on placebo, the researchers said.

Reinforcement learning is how we learn from feedback from our actions and environment, they said. "Our findings provide important evidence for the role of serotonin in reinforcement learning," said Christelle Langley, the joint first author also from Cambridge University.

"We are following this work up with a study examining neuroimaging data to understand how escitalopram affects the brain during reward learning," Langley said.