"This study provides preliminary evidence for potential therapeutic effects of simvastatin in the treatment of autism that warrants further investigations."
So said the results of the trial published by Ehsan Moazen-Zadeh and colleagues  looking at whether adding simvastatin - a lipid-lowering medicine - (20-40 mg/day) or placebo to a daily regime of risperidone - an antipsychotic medicine - to a cohort of "70 drug-free children aged 4 to 12 years old with diagnosis of autistic disorder" would have any effect on the primary outcome of irritability as measured by the "Aberrant Behavior Checklist-Community (ABC-C) scale irritability subscale score." A trial entry for this study can be found here (including reference "to evaluate the efficacy of L-carnosine in the treatment of autism" in the translated version for some reason?).
Anyhow, under "randomized, double-blind, placebo-controlled" conditions, researchers noted something of a potential effect from the addition of simvastatin to risperidone when compared with risperidone + placebo on the group irritability subscale scores over 10 weeks of use. They conclude that further study is required in this area.
There is very little other peer-reviewed science literature in this area of the autism research landscape. I note that Peter over at Epiphany has talked about statins and autism on a couple of occasions (see here for example) and how useful they have seemed to be for his son. Albeit based on an N=1, Peter speculated that the effect of statin use for his son (Atorvastatin) was not necessarily due to its effect on lipid lowering but rather other actions. Noting that simvastatin has specifically been linked to 'cognitive dysfunction' in some isolated cases  I understand that other studies have talked about a more protective 'pro-cognitive' effect from such statin use under other circumstances . I do wonder if there could be something more to see in this area pertinent to at least some on the autism spectrum.
 Moazen-Zadeh E. et al. Simvastatin as an Adjunctive Therapy to Risperidone in Treatment of Autism: A Randomized, Double-Blind, Placebo-Controlled Clinical Trial. J Child Adolesc Psychopharmacol. 2017 Jul 18.
 Suraweera C. et al. Simvastatin-induced cognitive dysfunction: two case reports. J Med Case Rep. 2016 Apr 5;10:83.
 Ling Q. & Tejada-Simon MV. Statins and the brain: New perspective for old drugs. Prog Neuropsychopharmacol Biol Psychiatry. 2016 Apr 3;66:80-6.