Preventive pharmacotherapy emphasizes the identification and reduction of illness risks prior to the onset of symptoms. The recent development of wearable biosensors has changed the technique by allowing constant, painless monitoring of biochemical and physiological parameters. This paper explores the integration of wearable biosensors into preventive pharmacology and their transformative potential for patient monitoring. Early detection of drug-related side effects, real-time monitoring of therapeutic effectiveness, and personalized dosage adjustments are among its key applications. The review emphasizes how artificial intelligence (AI), nanotechnology, and digital health ecosystems enable data-driven decision-making and precision pharmacology. Ethical concerns and implementation challenges are also discussed, along with future research directions for integrating biosensors into predictive healthcare systems.