Objectives: To summarize evidence-based pharmacological treatments and provide guidance on clinical interventions for adult patients with obsessive-compulsive disorder (OCD). Methods: The American Psychiatric Association (APA) guidelines for the treatment of OCD (2013) were updated with a systematic review assessing the efficacy of pharmacological treatments for adult OCD, comprising monotherapy with selective serotonin reuptake inhibitors (SSRIs), clomipramine, serotonin and norepinephrine reuptake inhibitors (SNRIs), and augmentation strategies with clomipramine, antipsychotics, and glutamate-modulating agents. We searched for the literature published from 2013-2020 in five databases, considering the design of the study, primary outcome measures, types of publication, and language. Selected articles had their quality assessed with validated tools. Treatment recommendations were classified according to levels of evidence developed by the American College of Cardiology and the American Heart Association (ACC/AHA). Results: We examined 57 new studies to update the 2013 APA guidelines. High-quality evidence supports SSRIs for first-line pharmacological treatment of OCD. Moreover, augmentation of SSRIs with antipsychotics (risperidone, aripiprazole) is the most evidence-based pharmacological intervention for SSRI-resistant OCD. Conclusion: SSRIs, in the highest recommended or tolerable doses for 8-12 weeks, remain the first-line treatment for adult OCD. Optimal augmentation strategies for SSRI-resistant OCD include low doses of risperidone or aripiprazole. Pharmacological treatments considered ineffective or potentially harmful, such as monotherapy with antipsychotics or augmentation with ketamine, lamotrigine, or N-acetylcysteine, have also been detailed.