Nasal sprays with the hormone oxytocin, nicknamed the "cuddle chemical", could ease the symptoms of schizophrenia.
Oxytocin is called the cuddle chemical because it helps mothers bond with their babies.
In the study, 15 participants used the sprays for three weeks only.
"It's proof of concept that there's therapeutic potential here," New Scientist quoted David Feifel at the University of California in San Diego, head of the team running the trial, as saying.
Each participant received oxytocin or a placebo for three weeks, then the opposite treatment for three weeks with a week break in between.
On the basis of two standard tests for schizophrenia, taken before and after each block of treatment, participants averaged improvements of around 8 per cent when taking the oxytocin compared with the placebo.
The effects didn't kick in until the final week, suggesting that it takes a while for the hormone to begin acting.
"Standard antipsychotic drugs increase their efficacy several weeks later too, so oxytocin fits that profile," said Feifel.
Feifel thinks that oxytocin is dampening down the excessive production of the neurotransmitter dopamine, which can trigger schizophrenic symptoms such as hallucinations.
He said that the rationale for treating people came from his own team's studies showing that oxytocin could relieve a form of psychosis in mice, and research showing that people who sniffed nasal sprays of oxytocin became more trusting, which could ease paranoia symptoms in schizophrenia.
The study has been published in Biological Psychiatry.