ADL5859 HCl is primarily used in research settings to study the role of delta opioid receptors in various physiological processes. It acts as a selective agonist for these receptors, making it a valuable tool for investigating pain modulation, emotional regulation, and potential therapeutic applications for conditions like chronic pain, anxiety, and depression. Its high selectivity helps researchers isolate and understand the specific effects of delta opioid receptor activation, contributing to the development of targeted treatments. Additionally, it is used in preclinical studies to explore its efficacy and safety profile for potential future clinical use.