The biphasic effect describes a phenomenon where a substance exhibits different, often opposite, effects at low versus high concentrations. This concept is particularly relevant in pharmacology and toxicology, where the dose of a substance can determine its overall impact on the body. In the context of testosterone and its therapy, the biphasic effect can manifest in various physiological and psychological outcomes. For instance, moderate increases in testosterone levels can enhance mood, libido, and cognitive function, while significantly high levels or abrupt changes might lead to mood swings, irritability, or aggressive behavior. Similarly, testosterone's influence on body composition and metabolic processes exhibits a biphasic nature; optimal levels promote muscle growth and fat loss, but excessive levels can increase the risk of cardiovascular diseases and metabolic syndrome. Understanding the biphasic effects of testosterone is crucial for clinicians when prescribing testosterone replacement therapy (TRT), as it underscores the importance of maintaining hormone levels within a therapeutic window to maximize benefits and minimize adverse effects.