The treatment developed by drug giant Roche acts on a brain receptor for the hormone vasopressin, which is believed to influence social bonding.
During the 12-week trial of 223 men, one group had the treatment and the other a placebo.
Those who took the drug scored significantly higher on social behavioural scales. Other research has shown positive effects on social skills from the drug.
In one unpublished study, 15 children had better scores after inhaling a vaporised form of vasopressin twice a day for four weeks.
Roche scientist Dr Federico Bolognani, who led the US adult trial, said: “We have nothing for autism. This is probably the best opportunity so far.”
Professor Declan Murphy, a leading expert in autism at the Institute of Psychiatry and London King’s College Hospital said: “If replicated by larger studies this may be a game changer.”
Dr James Cusack, of UK charity Autistica, said: “While medication isn’t appropriate for all autistic people, some on the autism spectrum want to be able to access medicines which can help them overcome the challenges they face.”