The purpose of this study is to test an investigational medication in children and adolescents aged 5-17 years old with irritability associated with autism spectrum disorder (ASD). The name of the medication being tested in this study is pimavanserin. This medication is investigational because it has not been approved by the FDA for the treatment of irritability associated with autism. The main risks of this study are from the medication, which include nausea, dizziness, and sleepiness.

If you are eligible and agree to participate, you will be randomly assigned to receive either placebo (a pill with no medication in it) or the study medication. Participation will last approximately 15 weeks and involve 8 study visits and one phone call. You will need to take the study drug for 6 weeks.