Study author Dr. Antonio Hardan of the Stanford University School of Medicine and Lucile Packard Children's Hospital said the pilot study involved 31 children with autism.
The study, scheduled to be published in the June 1 issue of Biological Psychiatry, found N-Acetylcysteine lowered irritability in children with autism as well as reducing the their repetitive behaviors.
Irritability affects 60 percent to 70 percent of children with autism, Harden said.
"We're not talking about mild things: This is throwing, kicking, hitting, the child needing to be restrained," Hardan said in a statement. "It can affect learning, vocational activities and the child's ability to participate in autism therapies."
However, Harden and colleagues emphasized that the findings must be confirmed in a larger trial before the supplement can be recommended for children with autism.
In a double-blind study design, children ages 3-12 received N-Acetylcysteine or a placebo for 12 weeks. During the 12-week trial, the treatment decreased irritability scores from 13.1 to 7.2 on the Aberrant Behavior Checklist, a clinical scale for assessing irritability.
Stanford is filing a patent for the use of N-Acetylcysteine in autism. One of the study authors has a financial stake in a company that makes and sells the N-Acetylcysteine used in the trial.