This chapter highlights that many nonpsychiatric drugs used for other medical conditions can cause psychological or behavioral changes. These effects can range from mild mood changes such as anxiety, depression to severe psychiatric symptoms like delirium, psychosis, and neurocognitive impairment. This chapter uses an organ system approach to detail-specific medication classes such as anti-seizure medications, Parkinson’s drugs, corticosteroids, sympathomimetics, anticholinergics, beta-blockers, angiotensin-converting enzyme inhibitors, antivirals, interferons, chemotherapeutic agents, etc. to illustrate their associated psychiatric side effects and possible pathogenic mechanisms. It also includes several interesting case studies illustrating complex drug interactions and adverse effects which are routinely encountered but often missed. Tables summarize common psychiatric symptoms of various medications, drug classes, and steroid comparisons. The chapter emphasizes the importance of close monitoring, especially in older adults and those with preexisting mental health issues, and the need for adjustments to medication dosage or additional interventions to manage these side effects.