Does it work?

So far, only one controlled study has shown that vitamin C can help treat behavioral problems associated with autism ⁽²⁾. This double-blind experiment showed that high doses of vitamin C in children with autism had significant positive effects on behavior, compared against their own behavior scores when they were taking a placebo. However, this study was very small (only 18 children), and has not been replicated ⁽¹⁰⁾.

Another research report (uncontrolled study) showed that high doses of vitamin C may help sleep and gastrointestinal problems in children with autism ⁽⁴⁾. In this study, vitamin C was taken along with other vitamins, so it's not clear which vitamin was responsible for the positive outcomes.