Paracetamol is an effective pain reliever but also reduces feelings of pleasure, a study suggests.

The previously unknown side effect means that over-the-counter painkillers are leaving users not only pain-free, but also emotionally numb.

In a study carried out by US researchers, volunteers who took paracetamol reported weaker feelings when they were shown photographs that were either pleasant or harrowing, compared with volunteers who had not taken the drug.

"This means that using paracetamol might have broader consequences than previously thought," said Geoffrey Durso, the lead author and a doctoral student in social psychology at Ohio State University. "Rather than just being a pain reliever, paracetamol can be seen as an all-purpose emotion reliever."

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Previous research had shown that the painkiller works not only on physical pain, but also on psychological pain.

However, the new study took those results one step further by showing that it also reduces how users experience positive emotions.

Dr Baldwin Way, assistant professor of psychology at Ohio State, added: "Most people probably aren't aware of how their emotions may be impacted when they take paracetamol.

"People who took acetaminophen [paracetamol] didn't feel the same highs or lows as did the people who took placebos."

Researchers asked 82 volunteers to take 1000mg of paracetamol - the equivalent of two normal tablets - or a placebo. They were asked to rate 40 photographs on whether the image made them feel positive or negative. The photographs ranged from the extremely unpleasant, to neutral, to very pleasant, such as children playing with kittens.

Participants who took the drug reported a less extreme reaction to all the photographs than did those who took the placebo. The same was true of their emotional reactions.

The research was published in the online journal Psychological Science.