Pharmacologically active substances are compounds that can produce a biological effect in the body. These substances interact with specific receptors or enzymes, leading to changes in physiological functions. They are commonly found in medications, which are designed to treat or prevent diseases. These substances can be derived from natural sources, such as plants and animals, or synthesized in laboratories. Examples include aspirin, which is used for pain relief, and antibiotics, which help fight infections. Understanding these compounds is essential for developing effective treatments and ensuring safe use in healthcare.