Agonists are substances that bind to specific receptors in the body and activate them, leading to a biological response. They can be naturally occurring, like hormones and neurotransmitters, or synthetic drugs designed to mimic these effects. For example, morphine is an agonist that binds to opioid receptors, providing pain relief. In pharmacology, agonists are often classified as full or partial. Full agonists, like heroin, produce a strong response, while partial agonists, such as buprenorphine, activate receptors but to a lesser degree. Understanding agonists is crucial for developing medications that can effectively target specific conditions.