Common painkillers such as ibuprofen, aspirin and paracetamol alter sensitivity to painful experiences, and make the user less empathetic, and affect the way the brain processes information.
The findings are 'alarming', said researchers at the University of California at Santa Barbara, and health watchdogs should be made aware of the potential harm that the drugs—none of which need a prescription—could be doing.
"Consumers assume that when they take an over-the-counter pain medication, it will relieve their physical symptoms, but they do not anticipate broader psychological effects," said lead researcher Kyle Ratner.
In a review of previously-published studies, the researchers say that the painkillers blur the distinction between physical and social pain. They don't just blunt pain, they can also "hinder people's ability to put themselves in another person's shoes, and feel that person's emotional and physical pain", the researchers say.