During clinical trials for olanzapine long-acting injectable formulation (Zyprexa Relprevv), a rare (1%, including two deaths) but serious adverse event emerged. PDSS occurs when the injection is inadvertently given intravascularly, resulting in an increased serum level of olanzapine. Symptoms most commonly occur during the first hour post-injection and include confusion, anxiety, dizziness, sedation, and extrapyramidal symptoms. Because of these instances of PDSS, a registry for this medication is required as is direct observation of patients by a healthcare provider for three hours post-injection.