How do drug molecules bind to receptors?

Drug molecules bind to receptors—which are specialized protein molecules—to initiate physiological or biochemical changes within the body. This interaction involves two primary components:1

• Binding: The formation of a drug-receptor complex. The tendency of a drug to bind to a specific receptor is known as its affinity.1
• Activation: The ability of the drug-receptor complex to produce a physiological response, referred to as efficacy or intrinsic activity.1

Drugs are classified based on these properties: an agonist has sufficient affinity and efficacy to bind and affect cell function, while an antagonist possesses the affinity to bind but lacks the efficacy to elicit a response, effectively blocking the action of agonists.1

The binding process generally follows the law of mass action, where the concentration of the drug influences the amount of drug-receptor complex formed. While many receptors are located on the cell membrane, some, such as those for steroid hormones, are found within the cell nucleus, requiring the drug to cross the cell membrane to bind.1